One example in action today, among others but this one you are part of: https://relay.basspistol.org it is essentially banning everyone, except those who have been invited. 😅
If you would set your client to only listen to that relay, no one from the outside of it could communicate with you. In many ways, I've come to assert that nostra is far more suitable for minorities at risk of bullying than fediverse will ever be, because the portability of nostr makes it much more resilient to external pressure.