I haven't been here long, but have seen this on other forums and as most of this site is visible to anyone I suppose if anyone said anything negative about a past member they might see it. Also is unfair as they can't respond to any negativity and would not rejoin just to do that (unless possibly unwell). They still have mh issues even if they have left.
I have made negative comments about past members on another forum, or at least inappropriate ones, and they could have seen those comments later and been pissed off, indeed I doubt I would have made them if they had still been around. That is cowardly and unfair, they couldn't have said anything about it by then and at the back of my mind I must have known that. So is maybe to protect them and anyone else from potential psychological harm.