I believe you would get the current keys. History would go back to the previous rotation and stop. For example. I ban user A. Then the community rotate keys. User B joins after the has ban happened. User B wouldn't see any of the conversation prior to the ban.

A strict client and group (say, whistleblower group) could pre-rotate so the new member gets no history, a lax client and group (a public community for example) could choose to only rotate on bans but not on joins, so joiners receive history but banned members are cryptographically severed. The default for current clients is rotation-after-ban, a nice default, but stricter works too. All of this is valid to the Concord spec, it’s designed to be very flexible, for critical privacy or public scaling, either way, fully encrypted, fully obfuscated. 🤝