Currently, we show "(unknown channel)" and "(unknown user)" just like that in the message-list app bar—lowercase and parentheses—the same way we show it in other places in the app:

Alya points out, in #1947 (comment) :

2. For "(unknown channel)", I think we can style the heading as "Unknown channel". Web also has a ? icon, but it doesn't look great, and probably isn't necessary -- I think it's still distinguished by lack of a channel privacy icon.

3. I'm not sure if we can also drop the ( around unknown user?