We should check to see if this happens with Iris in the Mixin version of Vivecraft. Would be nice if it didn't, but I have no idea.

Shaders work fine under Iris. Going to close this, as Optifine compatibility sounds like a mess that's not worth dealing with when a substitute already exists for all versions of Minecraft ImmersiveMC supports (except 1.16.x Vivecraft).

This is fixed as of 1.3.0