For current servers starting with zulip/zulip#29498 (server-10, ZFL 303), deactivated users are no longer members of any groups.

On the client side, the change this requires is that when we get a realm_user/update event saying a user was deactivated, we should remove them from all groups. See API docs.

Chat thread: https://chat.zulip.org/#narrow/stream/378-api-design/topic/Groups.20events.20for.20deactivated.20users/near/1958907