Is OptiFine allowed?

no

you can only use it for pre 1.15. 1.15+ is banned because it changes the behavior of the vanilla game and these features are non-vanilla unlike optifine 1.14-1.0 which doesn't change it and can easily disable these features if you are using 1.14-1.0 read the rules for optifine https://docs.google.com/document/d/1A7NtP7LegD7SYjho54gQDSlJkYZlofndjw2COu00yMU/edit#heading=h.pad3nic7pyya but if you are using 1.16+ use the mods here https://www.minecraftspeedrunning.com/public-resources/mods

thanks

1.16+ you cant

@Luminaire 1.15+

Only if you need h to e performance enhancement like if you are using a laggy computer/pc

^ this is incorrect