The problem with these efforts is always the same: organizations make their own messenger, and the fact that these organizations then have control over their own messenger ... means their employees won't use it. And that's ignoring that you can bet your firstborn they cut corners developing these messengers, so they're not pleasant to use to boot. In 2026 you still hear complaints of government employees that they on…
For private discussions, you do that on your private device, with a private messenger.
I would say that the digital sovereignty is more about "Entity X doesn't want the US to have access to all of their internal communications". Typically a non-US company or a non-US government should care about that.