Autosave should warn you if you open old presentations and you're going to lose slides
In a Powerpoint presentation I built last year, I opened it so that I can remove a few sensitive slides and save-as another version. To my dismay Autosave was put on by default and promptly crashed powerpoint when I tried to deactivate it. When I opened Powerpoint I could not undo and recover the slides I deleted. This is incredibly frustrating. The user should be warned if Autosave is suddenly activated on older presentations, or warn the user that slides will be lost!
Thanks for your feedback! Now, Office will warn you that AutoSave was on for your file if you make changes to cloud files and then Save a Copy to another version. To avoid situations like this in the future, we recommend using Save a Copy first before making changes.
As you know, with Version History, which you can access by clicking on the document name in the titlebar, you can restore any previous versions of the document.
Please note that AutoSave only applies to files on OneDrive or SharePoint. For files stored elsewhere, such as your local hard drive, changes won’t get saved until you manually save.
To learn more about AutoSave, please see https://support.office.com/article/What-is-AutoSave-6d6bd723-ebfd-4e40-b5f6-ae6e8088f7a5