I'm hoping to see the Alternate Origin GUI mod updated to 1.21.1, as I'm putting together a pack I'd like to use it in.
The original mod can be found here
There hasn't been any activity in the GitHub repo in the past 7 months, so I don't know if the original mod is still being worked on. It seems unlikely that it will be updated in the near future.
The mod doesn't seem to be too complicated, consisting of a single interface, a couple of mixins, and a texture for the GUI.