That effectively disables AutoSave - while also leaving you without the aforementioned backup. What you can do is turn off AutoRecover, the feature that automatically creates a backup at regular intervals. There's no way to globally disable AutoSave. It gets worse: Although you can easily turn off the new AutoSave feature by clicking the little toggle in the upper-left corner of the screen, that turns it off only for the current document.

To me it seems borderline insane that Microsoft would change a fundamental method of saving Office documents without adequately informing users. This is especially frustrating because the update was pushed out without warning, explanation or instruction. What's more, when working on a document that's already been saved at least once, you'll notice that the 'Save As' option - as old as the software itself - has disappeared from the File menu. Because of an incomprehensible change pushed out not long ago, any changes you make are automatically saved - meaning your original document is overwritten, even if you don't want it to be. Just one problem: Microsoft's Office 365 no longer works that way. That leaves the original alone, but gives you a modified copy. Here's a common practice: You open an existing document in Microsoft Word, Excel or PowerPoint, make some changes to it, then save it using a different filename.