I know that there are addons for Private Messages... but I want to say something:
Private Message System is too important to delegate to third parties. I am sure that these developers have created these messages-addons with all their best intention, but webmasters have very critical information in those messages to leave on hands of someone who is today... and tomorrow may be not, for different reasons.
I can assure you that many webmasters will not migrate to Flarum for this reason and they will go directly to Disclourse or similars just for this thing: Native Private Messages.
Please, reconsider it. We want to feel safe, we want update Flarum with security that everything will be updated correctly at once (and not waiting for the addon-developer in every F. update)