alt Hacker News> And yes, I'd call several mainstream religions cults because of this
Which ones ? Not something I have seen myself
I do like your definition. Its a pretty clear and meaningful distinction and linked to other cult behaviours such as encouraging members to cease contact with ex-members.
People I know personally were strongly ostracized after leaving Jehovah's Witnesses (in the US) and the Catholic church (in Latin America). Cut out of their existing social circles, immediate family members refusing to attend their wedding over it, etc.