Is your feature request related to a problem? Please describe.
Custom Item Textures and Custom Entity Models are Optifine options for texture packs that allow resource pack creators to add custom changes textures based on a text file.

Describe the solution you'd like
It would be nice if CIT and CEM could be implemented in Iris

Describe alternatives you've considered
Optifine has a working CIT implementation, but Optifine is slow and not open-source.

Additional context
The main reason Optifine is valued by map and resource pack creators is for its implementation of CIT.

If you want CEM, you can check out my mod

This is achieved by other mods and outside iris' scope