Comment by fleddr

You're right that the main source is a 2018 original tweet, and then some articles building on that tweet, and then news dries up.

This is the best I can find, with the Microsoft MVP clarifying the situation:

https://mspoweruser.com/no-microsoft-is-not-rewriting-office...

Specific point of interest:

"Office 365’s UI, a lot of it, but definitely not all of it, are pieces that are built using React Native (Windows). API’s and Services are still going to be powered by C++, C#, or whatever is the most appropriate for that team. Nothing is converting to “all/completely” JavaScript/TypeScript."

To me, this confirms that at least the UI is React. Anecdotally, it feels very "web" as I use it daily. Sluggish, missing paint cycles when switching between apps, and the typical main thread freezes.

I'll let you decide if this is convincing enough evidence :)