Why is OptiFine banned for 1.16+? I dont see why it would, because I think OptiFine does the same thing for each version? Thanks

Because of some technical stuff.

like

@AdvanceX I believe it has something to do with adjusting gamma - especially useful in the nether since the new update has brought about "fog" for the new biomes. So removing/reducing the fog would make it a lot easier to find fortresses, etc. See from SpeedNintendo: "Just optifine for 1.16 (at least for now), and we're banning optifine for VARIOUS reasons. One major one being that no matter what, fog in optifine is like 2.5x further than in vanilla. And no combination of settings can fix that. Providing a huge and unfair advantage. We can't just ban optifine on all versions because people with bad computers will complain." https://www.speedrun.com/mc/thread/vomey

It also changes how mob spawn

And there’s lazy chunk loading and fast math which is hard to detect

Ah, ok. Thanks.