Oh god please no. Apps that save automatically are the worst idea ever. I want to be able to make exploratory modifications, and then commit to disk AT A TIME OF MY CHOOSING. The computer doesn't know what I'm thinking, and it can't tell the difference between "messing around" content vs "serious" content.
I can see it now: I hit undo a bunch of times to get back to a previous state, copy some content that I wanted to bring back, then redo a bunch of times to get back, except I fatfinger it and hit paste by mistake. Now my whole redo path is gone, and I can't even reload to the state I was at before hitting undo thanks to the braindead program that "helpfully" saved my document behind my back.
Or even worse: I make some exploratory changes, then decide after changing a LOT of things that it won't work. The app crashes/the computer dies, whatever. I reopen the app and it's saved all the changes I don't want anymore. Maybe if I'm lucky it would have saved an undo path (unlikely), but even then, how far do I have to undo to get back to the sane state I was in?
Save means "I'm sure this is what I want. Persist these changes".