I'm a designer working on creating a prototype in Teams and would like to use Sketch. I've tried finding a Sketch file with all of the Teams UI in it but haven't found a complete sketch file for either desktop or mobile.
There is a Teams developer AMA coming up on the 14th here which may be a good opportunity to ask the question. However, the Teams UI is standardised across the web and desktop versions - so the response may not be favourable. However, I would encourage you to ask!
Finally, I would also raise the question on Github to ask the dev community too as they may have heard more through their contacts.
As you say a sketch file does exist but it is only things like cards, it isn’t the whole UI which again shows the limited ability to redesign the Teams UI. AFAIK, I don’t think that a sketch file of the whole UI exists for public consumption and redesign.