[tempfile]

I think you are exaggerating the importance of undo/redo. Even in google docs, if you restore from a savepoint you can only go backwards. And besides, the complaints OP got were "I'm losing all my work" - I doubt someone who had some recovery point would be in a worse position to recover than someone who had no recovery point at all.

(in other words, it's not necessary at all to support this feature from the ground up - it just might be necessary in order to have a third, even better way than "no saves" and "imperfect saves")

[Ferret7446]

Well, going forward is undefined (i.e., redo history loss), unless you implement "linearized" history [1], which anecdotally is very confusing for most people.

[1]: https://www.felesatra.moe/blog/2019/08/04/emacs-undo