Is your feature request related to a problem? Please describe.
As far as I know there are no animation variables for the elytra models, are there? I have not found a way to change the elytra's behaviour when you stand, shift or fly.

Describe the solution you'd like
As far as I know there are two factors to the vanilla elytra rendering, which is IF the elytra is folded out in the first place, but also, if it is, by how much folded forwards or backwards it is. It would be cool to have the ability to have this as animation variables.

Does OptiFine do this
No, since OptiFine does not provide a way to remodel the elytra.

Additional context
Am I making a mistake and is something like this already implemented? Then please add it to FEATURES.md.