JTL Flarum PM's was fine, maybe could've used a bit more distinction between forum posts or PM's
It is functional, but also buggy. During upgrades I've had to reset the "is_private" flag directly in the database to rehide private discussions - for all private discussions.
As for design, as long as you regularly logged in it worked. If you don't, the private discussions can easily be overridden by others and become lost. There are still no notifications for this either. Then there's issues like inability to allow it to become a public discussion as well as "new discussion" notifications that are actually private discussions that you can't see.
Some of these issues I brought up in 2015-2017, but some never were addressed and the design was established as a normal discussion, just hidden from most people.
There is some work on a more legitimate pm extension, however it looks like it'll be for cost. And the trend seems to be some kind of monthly payment.
JTL Also I notice the "profile" filed of user accounts doesn't exist anymore. This intentional?
I think that was related to a forum upgrade where the dev's removed it. There was a extension to replace it, which I've installed.