I have this in my mod folder, and while the game loads up fine, none of the model changes I've made that depend on this appear to manifest. Installing this alongside Mod Menu doesn't show it in the menu. I can't seem to find any errors in the log implying a failure to load this, so I have no idea what's going on.

can you send the game log (either as a file attachment here or by pasting it to some paste site?)

I'm not seeing anything that would imply a failure to load in the log.
latest.log

This still appears to be affecting 1.21.

latest.log does appear to raise an error somewhere in the log:
latest.log