I have discovered a fair few glitches/glitch variants over the last month, sharing progress on Discord, and would like to share them on the forums for future reference of anyone who bothers to check.
(My discord username is currently Overturn, to clarify)

These are all almost exclusively useful for Extra Spicy, and the bootless routes in particular.
However, Blackout A may be useful to save time in Normal any%.

1: Extra jump glitch, checkpoint variant (cphop) - By saving and quitting the game while being within both ground and wall grace periods, you can jump twice (three times with boots) in midair as you respawn, as long as you time the first jump early enough. (With only one of the two grace periods active, you can perform one of their respective jumps, but no extra jump.) This is what I used to climb out of the "Lose" room in Extra Spicy Bootless 100%.

2: Extra jump glitch, wall-only variant (draghop) - By pausing and unpausing the game and, while unpausing, dragging the game window in windowed mode, while hugging a wall with gloves, you can freeze the game in an unpaused wallhug state. By holding down a jump button and pausing again, you can walljump "for 0 frames" before re-pausing. Repeating the unpause-drag-hold jump-pause setup causes an extra jump to be stored the next time you unpause. However, the first "0-frame jump" is of minimal height, making some jumps slightly harder than they should be. This is what I used to cross Abstract Bridge and climb multiple rooms in Extra Spicy Bootless 100%.

3: Blackout, snq cancel variant (Blackout A) - Clicking "quit to title", then saying both "yes" and "no" at the same time (achieved by either pressing enter+esc or by clicking no while hitting enter on yes) will cause the game to resume, but in title screen mode. In other words, every entity on screen is unloaded (includes moving/breaking platforms, bags, checkpoints, moving enemies, bullets), the player character becomes invisible, and every map tile becomes invisible. (Furthermore, this leaves the cursor on-screen, allowing you to drag the screen during normal play. The effects are the same as dragging during an unpause.) Leaving the screen reloads entities, but everything else remains black and tangible. This is used to skip otherwise-required checkpoints, and potentially to pass through breaking platforms from below. This can be used to do Extra Spicy bootless orbless any% (15.28 low%). In recent news, since this allows you to keep your mouse cursor on screen when unpaused, you can load a game while saving (see save manipulation teleportation).

4: Blackout, main menu double cancel variant (Blackout B ) - Caused by clicking "back" on an 'order-1' main-menu option (1 esc press brings you back to main menu) in more than one way at a time. Effectively continues the game exactly where you left off, without moving you or changing your state other than entering Blackout. Same effects as var A from then on. This variant cannot be used for save manipulation teleportation because your mouse cursor is not kept on screen.

4.5: Main menu double activation - Click one option while pressing enter on another, and you will activate both. Won't go into detail because it's pretty much useless.

5: Extra jump glitch, wall climb variant (climbhop) - When you jump against a wall with gloves, there is a short time frame when you can press jump again without jumping. Doing so will store an Extra Jump. This can be chained together to climb a wall entirely vertically, which is substantially faster than normal, and to transport an extra jump upwards. This may seem trivial thanks to draghop, but since this allows you to get two full jumps, you can actually do more with this version. In particular, Extra Spicy "Abstract Bridge" bootless orbless. (The full bootless orbless 100% route is missing 3 screens and 3 bags currently, even after taking this tech into account)

6: Extra jump glitch, default respawn variant (reloadhop) - When you reload the game by closing and reopening, you have an extra jump stored by default (only an extra jump, no corresponding ground or wall jump). This is potentially useful for Extra Spicy Tower of Dreams redless, which, if doable, would allow the player to get spider gloves without saving. However, you don't seem to move as fast when this is active, forcing you to touch the ground even with all current known tech... This is the only known way to get an Extra Jump without gloves.

7: Save manipulation, mode swap variant - With 2 windows of the game open, you can save the game in one game mode and load it in another. This is done by saving on the target game mode, quitting to title, using the second window to save on the other game mode at the target position, and loading the game on the first window. All stats are maintained except for game mode.

8: Save manipulation, teleportation variant - To perform a teleport, Blackout A on one window with your intended stats (items, percentage, time). Position your character near a save bell. Then, on your second window, save and quit to title at your target position. On the first window, walk toward the save bell and immediately click continue on the second window. If done correctly, the save bell will be hit while loading the save. This acts exactly the same as mode swap, but also changes initial spawn location, effectively teleporting the player. This should be safe for IGT runs in theory, as it never decreases any stats, but in practice it can be used to teleport to the final rooms before the win orbs in either game mode with no items and only 2 rooms explored, allowing for absurdly low completion % and IGT.

X: "Slomo clip" - Having slomo >= 2 active (best at 7-10), a small bit of lag, and some luck, you can clip through the ground at any (?) horizontal screen transition by changing screens right before hitting the ground. Past hypothesis: if your last frame of movement before a screen transition gets you far enough off screen, you can change screens at below ground level by going around collision. Current hypothesis: I have no clue. It defies explanation. Please help.

Please tell me if there was a better place to put this information, or if I made any mistakes in my explanations.

If a moderator has seen this then can I please get a verdict on the use of blackout glitch in IGT runs? The in-game timer is paused while attempting the glitch, but doesn't reset it if performed correctly. So technically it's no different from pausing the game mid-run, which has been allowed in the past.

The glitch is useful in "hardcore prawn", by passing through the crumbler to enter "point of no return" earlier. here's a video

On the topic of Blackout legality: as far as I can tell, neither variation of the glitch actually affect in game time. Even if you fail to perform Blackout A and return to the title screen, you can perform Blackout B and continue exactly where you left off.

Despite having returned to the title screen, doing Blackout B should not reset the timer either (if my understanding is correct) and would therefore ALSO be equivalent to simply pausing mid-run.

Which forms of the glitch are/aren't legal?

the problem with Blackout B is your time is reloaded from the last save (bell touch). It'd set the timer back, making the run segmented technically. This is why IJG cannot be used in normal runs for example

Ah, that's disappointing. Are you certain that's the case?
I don't see why it would be, because it doesn't load a save, but if the game actually does reload your timer after Blackout B, then I guess it can't be helped.

sorry I believe I may have misunderstood you, I thought you meant doing blackout B from the main menu

Yes, that is correct.
Blackout B doesn't load a save, even though it is done from the main menu. It simply continues the game where you left off.