Scroll down a bit to the "Feature support in particular profiles" table. It shows what techniques are allowed in each profile. This would pertain to things like trying to set custom encoder parameters (though most of the ones you would edit to improve quality for the typical usage scenarios in OBS aren't really affected by this all that much)
The only differences seem to be these four things which High supports and Main does not:
4:0:0 (Monochrome) - Irrelevant for us
8×8 vs. 4×4 transform adaptivity, Quantization scaling matrices, and Separate Cb and Cr QP control. Which I'm not sure which x264 features they correspond to and if they're even active in the presets that are commonly or uncommonly used by OBS users.