Restricted membership groups are still valuable, no matter what you want to call it.
Shared experiences are often a good foundation for a group: residents of a particular neighborhood, alumni of a particular school, members of a particular family, etc. You can see lively discussion there that opens up in a way that might not happen in a general open group.
Common beliefs also form a good foundation for group membership. Almost every religion has meetings of other members of that religion, where discussion can happen within that framework of that religion’s views. A Baptist bible study group wouldn’t tolerate a new member coming in and just insisting every meeting that the Bible is fake and that Christianity is a lie. Does it create an “echo chamber” of only people who believe in a specific religion? Well, yes, because that’s the point, and why those members choose to congregate there.
Hell, I’m in a sibling chat thread where specific members of my family feel safe talking about their struggles with their significant others, roommates, jobs, neighbors, etc., because we like being able to bounce ideas off of people raised like us, by the same parents, in the same household. I don’t think we’d be able to have that productive conversation if we didn’t have that specific thread that we knew was just for us, and not for the other people in our lives to read and comment on.
Unless you’re taking the radical view that people shouldn’t be allowed to congregate in smaller groups that restrict membership, safe spaces are a natural consequence of how people associate with one another.