[bni]

Does this mandate allow me to use a. 3rd party Teams, Google Chat and Slack client?

I suspect the answer is no, but why?

[usr1106]

There are criteria for how dominant a platform is to be considered gatekeeper. Teams, Google and Slack have much smaller market share for private messaging, so I guess they are not affected. Don't remember the criteria by heart.

[benoau]

They are not designated gatekeepers. It is unfortunate because interoperability should be its own objective for its own sake.

[aniviacat]

Because they're not big enough to be considered a "gatekeeper".