In many modes, Blender uses the `MemFile` undo step, which serializes all DNA
data in RAM almost as if writing a .blend file. For auto-save, Blender used to
write the last `MemFile` undo step to disk because that was faster serializing
all of DNA again. Furthermore, saving the `quit.blend` file when closing Blender
also used this.
This functionality is now removed in preparate for supporting implicit sharing
in the undo system (#106903). Auto-save and saving the quit.blend file now use
regular file saving.
The removal of this feature and its implications have also been discussed here:
https://devtalk.blender.org/t/remove-support-for-saving-memfile-undo-steps-as-blend-files-proposal/33544
efb511a76d