I think admin actions like impersonation are logged.
Anonymous mode wouldn’t work because it’s a private category only accessible to a particular group (plus admins).
Admins can also read PMs - they have access to everything. In any case, we prefer categories and topics to messages.
I’m not really too worried about all this and don’t want to create draconian or big brother-esque oversight of admins, but thought it would be worthwhile to have a way - even if it’s hard to do - to check in case there is ever a concern.