‘It wasn't a space where no bad thoughts or views could be expressed, but a space where people were free to make mistakes, to very clumsily summarise the theory (I'd welcome a better short description if anyone has it!).’
It’s a therapeutic term that I think means a space with clear boundaries where difficult things can be discussed with consent, What’s happened to it is similar to what’s happened to ‘sitting with discomfort’. The idea isn’t meant to be that someone causes you discomfort and you have to sit with it (being ‘held accountable’ as a form of ritual punishment). It’s meant to be that one way of dealing with problems is to learn not to bury/repress/medicate them away, and learn to co-exist with the discomfort they cause you.
But like ‘safe space’ that idea has been twisted, purposefully, to justify terrible behaviour towards other people.