Can I use optifine 1.16.3 for my speeedrun or no?

Assuming you talk about Minecraft, you should ask in the Minecraft forums: https://www.speedrun.com/mc/forum

If not, go to the forums of the relevant game as well.

Read the game's rules. They cover this.

Can we please make a pinned post in some of the forums describing what the forum is for and with the words "Game specific questions should be directed to their respective forums"?

@Lietenant_Boo like anybody would read it lol

But yeah maybe one pinned topic with a clear heading could help.

Doubt it'll make a difference at all, but I made one anyway https://www.speedrun.com/speedrunning/thread/6tgel

Much appreciated

Tbh I'd have mentioned Minecraft by name in that post. But thanks for it, it certainly can't hurt.