I have been getting into speedrunning in the Minecraft community and I've now been working on trying speedrun. However, I have heard that Optifine cannot be used during a speedrun or something down those lines. Plus, most of the runs in 1.16 are Jellysquid or vanilla. I just want to confirm because I don't want to submit a run with Optifine and getting rejected just because of that.

you can use optifine in pre 1.16 but not in 1.16+