Well that's disappointing. The simple ribbon in the new OneNote feels like such a step back from the ribbon UI in OneNote 2016.

It does say users will be able to enable the full ribbon UI still, so I suppose I will have to do that. Hopefully they don't deprecate that feature.

In the past the reasoning to replace the menu system with ribbons completely and not allowing to switch back is the understandable desire to not having to maintain and test two different UIs all the time. Two different ribbons may still work, as the ribbon already has information about "importance" or "prominence" of each item, but I fear eventually there'll be only one remaining in the future.