Really being able to control PMing between TL/Groups would be the ultimate goal, but I know that’s not possible, so the next best thing was restrict creating PMs say at below TL3 but then disabling replying also for anyone <TL3.
So a PM conversation can not be instigated by someone =>TL3 with someone <TL3 and continued. I understand the need for admin/mod PMs to PM restricted users and more than likely requiring a response and so see the design intentions.