If I'm not mistaken, Microsoft's chat apps were never part of Office. That's the issue at hand. Microsoft is not forbidden from making competing products, it's forbidden from misusing a dominant market position in A to seize the market in B.

Office Communicator came out in 2007. Hard to say "Office Communicator" has no relationship to Office. That's six years before slack's initial release.

According to the top level comment in this thread those chat apps were bundled in office.

user: llimos > there has been a chat app as part of Office since 2007

I'm old. I've used every single version of Office both at home and in the Office and none came supplied with a chat app. The only exception at one point being Skype for Business, but I'm fuzzy as to whether this was a default or optional package.