the fact that these organizations then have control over their own messenger ... means their employees won't use it.

Not sure what you mean here; I happily use whatever work email and messenger systems are provided for work. Most people do. I don't actually mind that IT services have access; they are in any case covered by GDPR.

In some cases there has been a legal crackdown on back channels: https://www.ft.com/content/68c26cf6-52d5-11e3-a73e-00144feab...

The Boris Johnson problem remains, but it can at least be made against the rules for normal work purposes.

(Remember not to type crimes into a computer, people)