Letting admins (and optionally mods) read private messages can be useful. But most of our users don't think it is a good idea, so it is better if they don't know and there is no way for admins to accidentally let everyone suspect that such a feature exists.
One of our competitors has recently lost more than 50 active users just because there were a proof that mods can read private messaging threads! There were a real scandal - it was read by many users; it all ended up in users leaving the forum (some of them have registered with us).
We do not like the idea of our mods reading PMs, so we switched it off. However, while we, the admins, rarely read PMs to understand what's happening behind the scenes when there is a hot discussion, I noticed that I can do any of this: "like", "share" and even "invite" others to the discussion. In my view, this should be OFF - admins should be able to read in READ-ONLY mode (strict), with no way to accidentally do anything that would disclose such a feature.
Letting everyone know about such a feature will not work - partially because of our mentality, partially because Russian/Ukrainian-speaking people do not trust each other as much as people in other nations do (not sure which ones?). Thus, it is not an option nor a partial solution for our forum. It is not possible to adequately explain to our users that whatever day do, it is a normal feature that we always can read everything and anything right in the database. Most of the users strictly believe that PMs are really hidden from everyone, including admins. It is simply not the right time for our nation to think and understand that who owns the forum also has access to PMs at any time.
If not as a default feature, it would be very helpful to at least be able to SWITCH OFF any edits/actions that admins can do while reading PMs where they do not participate personally.