I can confirm that doing a quick search for ProxyTexture on Github, returns no results besides the files that define ProxyTexture.
I did a search through the Godot 2.1 source code, and it also returns no results beyond the files that define ProxyTexture.
I did find a single issue that mentions ProxyTexture, but outside of that issue and one closed issue, over all of the time Godot has been on Github there has been no other issues mentioning ProxyTexture.
From what I can tell, it appears that ProxyTexture is supposed to be used as a base for making other textures.
If it is supposed to be used as a reference though, maybe it should be moved to the documentation or something? It seems like it would be better for it to be somewhere like in the documentation than unused in the source code.
Without knowing for sure, I would say it is probably deprecated and should be removed. It does seems rather strange that it is not anywhere.