Snapshots are stable and supported. But currently there are only few helper functions that make them actually usable for you.
The editor bindings (well, at the moment only y-prosemirror) make snapshots rather accessible. But the Yjs types currently don’t export methods to revert to an old state.
Documentation is still a WIP. I started docs.yjs.dev (very experimental). And i will also provide a better, more accessible API for versions and showing the differences between versions. But I need time to finish this (and many other milestones) up.