I think the admin always has the power to see things in all tools unless it's true E2E encryption which I think is very rare in corporate environments, or even non existent?
More broadly, end-to-end encryption may put the employer at risk in some situations. In harassment claims, employers can become liable for harassment using company-provided channels because they're argued to 'enable' it. Providing a communication channel that doesn't allow them to verify claims during an investigation or stop ongoing harassment can expose them to greater risk.
To my knowledge, in Europe (or some European countries) it's illegal to monitor employee communications except in the context of a specific claim. In the US broad-based communication monitoring is generally legal unless someone can somehow prove that it's specifically used to prevent employees from organizing a union.
IANAL, take these as broad-stroke general impressions and not precise or accurate statements of fact.
... with all necessary and legal steps taken regarding your continued employment at that organization should you refuse to voluntarily divulge the contents of the message or a way to decrypt it.
This of course depends on the structure of the communication, but for many industries, it is required by law to be able to produce communication upon request, and not doing so would be a crime.