Maybe a notification saying "[User] forked you! (It's a compliment, mostly.)" with a button to view & adapt?

If all MLS group members keep the previous root key, then when a member who has not updated to the new root key continues to encrypt messages using keys derived from the old root key, members who have updated to the new root key can determine that someone has forked off from the group and can privately notify that person. Of course, this approach can only detect members who send messages; it cannot detect members who only read messages without sending any. In addition, it sacrifices some degree of forward secrecy. This is just our temporary idea, and we have not yet developed a concrete solution for this problem.