Given Nothing and Sunbird’s small marketshare, I assumed Apple was probably going to let Sunbird slide. Now I hope they bring the hammer down.
E2EE is one of the main points of iMessage. Security minded iMessage users are not going to feel comfortable if a Sunbird user is on the other end.
Sunbird / Nothing needed to play this very carefully. Users are giving them their iCloud credentials and are technically giving them the ability to view encrypted comms, documents, photos, password wallets, health records, etc.