In this view the save action is more like a commit, where the user manually checkpoints and also offers a simple description of why this is an important point. But in my view all intermediary points also need to be saved, because the user might have forgotten to explicitly checkpoint, and might still want some undo/redo capability that is more granular than just checkpoints.
Auto-save is nice, but I think it works much better when it's a separate file.
This way "main" document file (which might be checked in to git, or shared via dropbox or read by some document) only contains nice, clean, saved version.
And yet if your computer crashes for whatever reason, the data is still not lost and can be trivially recovered.
but the save button could simply tag specific save points in a larger table.
if the format can roll up changes to compress them, they also indicate where which variants need to be kept indefinitely.