I'd like to open this discussion to better define the glitchless category by evaluating any questionable or unique strats.
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Currently Banned Glitches:
Prologue Skip
Delayed Swandive Jump
Out of Bounds
Crouch Bug / Grenade Fall Cancel
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Here are a few strats to start off the discussion:

1 - Floating Grenade
- Info: Throwing a sticky grenade while looking downwards with crosshair, having certain weapons equipped, and a high FPS will stop it mid-air.
- My Thoughts: Glitch - A grenade should actually be sticking to something to use it in a glitchless strat.

2 - Slope Kick
- Info: A running kick allows Lara to ascend a slippery slope.
- My Thoughts: ? - The overlapping animation and the upwards distance gain are the only points I could make.

3 - Major / Minor Clipping Issues
- Info: These are examples of Lara's model clipping through objects without going out of bounds.

Major:
A - Burning Debris
B - Niflheim Rock 1
C - Niflheim Rock 2
- My Thoughts: Glitch - Lara's model is freely moving through something that is perceived as solid, even if it is not. Examples A and B can be done with no/minimal visual clipping.

Minor:
D - Tiger Skip
E - Crypt Ledge
- My Thoughts: Not A Glitch - The only visual clipping happens during animations that lock Lara into them. While you can make an effort to avoid both of these examples, they don't give any more of an advantage.

4 - Thailand Statue Opening
- Info: After lighting up both small statues there is an unskippable cutscene of the ground opening up. The flat collision instantly turns into stairs while the textures descend slowly. If you have both small statues in place before the second jewel lights up it will skip the cutscene entirely and you can fall right through the ground and proceed.
- My Thoughts: ? / Leaning Towards Glitch - The setup is arguably natural, but Lara's model fully moves through ground textures and can see out of bounds even though she technically is not. It's honestly just a weird looking strat. If we decide that it is a Glitch, I think the best thing to do is to disallow the cutscene skip so that Lara can't move until everything is in place. I feel this would be a more direct way of handling it.

5 - Jan Mayen Bike Fall
- Info: As you can see, short falls on the bike will kill Lara. Collecting at least two relics to increase max health makes this strat possible when landing in front of the Gate of the Dead. I believe this is because that spot is a checkpoint / cutscene area that does actual health damage instead of the instant death you would normally get on the bike.
- My Thoughts: Glitch - I view this as a form of cancelling / minimizing fall damage. The fall distance is extreme and the results are unnatural.
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I would highly appreciate any and all feedback, even if you just wanted to quickly list off the strats and a quick "Glitch / Not A Glitch" opinion.
Please feel free to bring up any new strats that you find or anything that you would like clarification on, no matter how small. That's what this thread is for.

1 - Floating Grenade: Glitch.
2 - Slope Kick: if that kick was enough to reach the very top of that ledge than I would say not a glitch. But since you are running (even tho it's only for a little bit, (and you could argue that IRL it would also be possible to run for a second or two before you start to slide down, so it's "natural")) up the slope, which wouldn't be possible in normal gameplay, I'd say it's glitch.

But that brings me to the point of Swan Dive into slope in Mexico Trials hub. It's also climbing up the slope of sorts, and it looks even more unnatural with roll after the swan dive allowing you to ascend it, which would normally slide you down if you just rolled into any other slope, I suppose?

3A - Burning Debris: Glitch.
3B - Niflheim Rock 1: Glitch.
3C - Niflheim Rock 2: extreme Glitch.

3D - Tiger Skip: not a Glitch.
3E - Crypt Ledge: I would say not a glitch, but very close to it, since it almost warps you up there.

4 - Thailand Statue Opening: Glitch. I think waiting until cutscene ends is a bit pushing it, but waiting until certain moving piece of ground is fully stopped before touching it - yes. Except maybe for the last one with a hole, that one is probably fine to swan dive over.

5 - Jan Mayen Bike Fall: Glitch. While I think that collectibles to increase health are a good use to survive some sketchy falls/jumps, this one looks as unnatural as it gets and beyond glitchless.

What about swan dive off a red pillar in Thailand? On the upper floor of the room where you kill the tiger with shotgun. Setup itself looks normal, but she seems to not being able to reach the ledge (starting to fall down lower than ledge level) then in a moment she gets warped (moved up a bit? gain some upwards momentum by swan diving into the wall?) to grab the ledge.

The Mexico slope is definitely weird but I would attribute that to bad level design. It does not behave like a normal slippery slope, the bottom allows you to stand on it normally unless you walk forward (instead of jumping to the wall pegs) and then it starts dragging you upwards. I do not see this as an issue.

As for the red pillar jump, I would compare the weird locking animations to the Out of Time fast cycles or these "flailing" jumps that you can see here in Bhogavati and here in Jan Mayen. In my opinion these are not glitches because we are making use of Lara's auto-grab (existing mechanic) and bad level design, with the only "odd" thing being the look of the animations. I don't see any of these as a glitch either.

I'm technically not a runner so if you guys feel like my opinion doesn't matter that's fair enough
I have played the game quite a bit too so here are my thoughts:

1 - Floating Grenade: definitely a glitch. It's hard to avoid but just no, it can't not be a glitch.
2 - Slope Kick: it is unintended for sure, but I'm not entirely sure if it classifies as a glitch. Still maybe leaning a bit more towards it being a glitch though.
3A - I mean come on (glitch of course).
3B - If the player avoids actually going inside the rock textures then it shouldn't count but that would be hard to do constantly and that would make going for that small timesave tedious. So going inside the rock textures: glitch, simply jumping up and going around the textures without collision: should be allowed (if the player is willing to risk it).
3C - Absolutely shouldn't be allowed.
3D - Doing this with a swandive looks completely normal/fine. The regular jump strat looks incredibily glitchy to me though. That said, again, it would be tedious to specifically go for the swan dive and reset if you get the regular jump.
3E - I'd say that's fine.
4 - Should probably avoid this yeah.
5 - Yeah I vote for glitch too.
Red Pillar - Definitely fine in my opinion.
Flailing Jumps - They look really weird, glitchy even, but it can definitely be argued that it is technically not a glitch for the reasons mentioned above.

What about kicking off of a ledge? Example:
I used this strat too when playing the category (even at the end of prolouge) but it seems a bit wrong, seems like it messes with momentum but could be intended for all we know with this game (or simply just not intended but also not a glitch).

Personally I tend to be rather lenient with tricks before considering them glitches. I especially try to distinguish them from things that are unintended, but still within the realm of normal game mechanics use. That's why "this looks unnatural" or "this looks unintended" usually is not enough of an argument to classify something as a glitch for me. That being said, I'm not a glitchless runner, which definitely makes me biased towards not seeing something as a glitch.

Floating grenade (1) I also see as a glitch - the specific setup needed (where the angle you throw the grenade at and the equipped weapon determine if the grenade gets stuck in midair) as well as the fact that the behavior changes when having V-Sync enabled (or disabled), makes this a pretty clear case for me.

The slope kick (2) I do not see as a glitch. The analogy to TRL/TRA's rolls or backwards handsprings up slopes has already been brought up, as it seems like the animation of the maneuver takes precedence over getting pushed down. Lara being able to pull out her weapons to get some more distance is questionable, but you can neither chain kicks into kicks or any combination of kicks and weapon toggles past the first transition of kick into weapon toggle. After that first kick and weapon toggle, you're going down, and consistently so. That to me makes it more of an exploit rather than a glitch as it is a case of animations overlapping/being prioritized.

Clipping I don't see as a glitch in general, but would consider it as such if it allows you to bypass or pass through otherwise solid objects.
3A I don't see as a glitch - yes parts of Lara can clip into the debris, but you still have to jump over it and can't just pass through them. Those part you clip into never have collision, the texture doesn't match up with the collision. I think you can encounter numerous examples of such clipping issues in many spots in the game.
3B I see as a glitch if performed as in the second attempt in the video, because I think you "pop" out of being inside the slope. In the first attempt you're not clipping into anything solid from what I can tell. However, the extreme, almost pure upwards boost at the start of the swandive may be grounds for classification as a glitch, which I have no strong opinion on. Generally I don't see a problem with getting smaller boosts off collision, though cases like these are definitely debatable.
3C I see as a glitch because you can pass through an otherwise solid object. Note: If there was no collision on that rock, only the texture and you could just run through it, I probably wouldn't consider to be a glitch, to further demonstrate my line of thought. Same point as I tried to make with 3A.

For the Thailand statue thing (4) I have no experience to rely on. If the collision in fact does instantly transform from flat ground to a stairs shape, then it's more of a case of a texture mismatch, which would probably mean I wouldn't see it as a glitch. If Lara is physically transported "inside" the collision and you could "pop" out again, I would see it as a glitch. I don't know which one of the two applies.

I agree on the bike fall damage cancel (5) being a glitch as it is a way of altering normal game mechanics such as fall damage with a specific setup and requirements having to be met for this to work in predefined spots.

As an addendum, there is also always the option of having a "No Major Glitches" category or similar where you only disallow the more egregious and obvious glitchy things, while you can use Glitchless or whatever you want to call it to be more restrictive.