I've started to use the OneNote Web App, and I think Microsoft have made a very good job developing it. I use it on the computer and on the phone with ease. I use the keyboard shortcut ctrl + S to save and expect the document to be saved. After using it a while, I started to browse the ribbon navigation bar and found that the save-button is omitted.

Pressing the question "Where's the Save Button?" gets you to a popup that does explain why its gone:

That’s interesting. Especially since we on the User Experience Q'n'A at Stackexchenge often answer questions with conventions. As in the famous save icon question [1] where the accepted answer referred to an idiom.
However, I want to know which usability research Microsoft based there decision on to omit the save button. I've searched but didn't find anything. Not even on Microsoft official OneNote blog [2], where one would expect to find background information. This leads me to asking:
Based on what usability research did Microsoft omit the save button on OneNote Web App?