It is deeply offensive to see this tactic repeatedly used to insult people.
Anyone who genuinely has a mental illness of any sort who reads it feels diminished as a person even if it wasn't directed at them nor references their particular illness.
It reinforces prejudice and bias against people who have a mental disorder. You don't use cancer to insult someone.
I have a specific recent example but I won't use it because I don't want it to become about specific posters. I hope we can all acknowledge it happens and it is very wrong.
In my opinion it needs to be specifically addressed in policy and it needs to be an absolute prohibition.
No one should be allowed to make that mistake more than twice. One warning and then you are out.
At the very least anyone who sees it should call it out.