Well, you might know me as a mod over on that there forum being discussed. Only since March, mind you, but nevertheless ...
The most recent thread disappearance started like this ... I got a PM from the accident pilot asking that the thread be removed as he/she was still "recovering". I told that person that I would pull it for the time being, but no more than a week, and would have to discuss with the other mods. Other mods received some PMs suggesting that there was reason to leave the thread for discussion and so it was returned to service, so to speak. In the meantime, someone had posted a "what happened to the thread" thread. Then it disappeared with no trace in the logs. My guess is that admin received a PM, maybe even a lawyered up PM, demanding removal. The rest of the mods don't know what happened to that thread. Admin has no obligation to tell us, but may do so when he gets a chance.
Other threads referenced here which have disappeared have gone because someone did, in fact, lawyer up. AvCanada does not have the money to fight in court for the words of anonymous users who may not even have their stories straight.
AvCanada has a different moderation style, to be sure, than Pprune. Not better or worse, but different.