I see most of the runs are done in js0.23 (the web version, a.k.a classic.minecraft.net), however there is actually an issue with this - Mojang have updated this multiple times over 2019 and possibly even 2020, changing random things, and I believe this may include the physics, which would affect speedruns. I can confirm that it atleast has been updated over time as I have downloaded multiple archived versions of js0.23 from Omniarchive, and they all have peculiar differences (for example, later versions appear to have a glitched Steve model).

On top of this, I also see that the original 0.0.23a_01 is allowed. However, this version has completely different physics and gravity to the Javascript remake. Shouldn't there be some guidelines on what version of the game to use?

I would suggest making the original version of 0.0.23a_01 the only allowed version of the game to use, as the web version is actually slightly renowned in the classic Minecraft community for being buggy and a bit of a poor effort on Mojang's behalf, not to mention the inconsistencies between updates. However, I also understand that trying to install such version in question, and installing a launcher to actually run it properly (like MultiMC) might be a bit of a pain for newcomers, making the Javascript remake much more accessible to play.

Any thoughts on this?

yes maybe use a separate game for the separate versions this one would be web and java would be a entirely different game

"and possibly even 2020"

It's been a year and they haven't fixed multiplayer so I doubt that they updated it in 2020. (still possible because maybe Mojang is just too lazy to update their servers)

regarding the rest of your passage, I sorta agree with some stuff here and there

Every classic run has been performed in the web version, it would be dumb of us to delete every run on the page.