But EU interoperability laws are cancelled out by DMCA (aka EUCD). That’s why reading a DVD with VLC has always been “technically” illegal.
If Apple is able to update the protocol in such a way that it requires some kind of signed attestation from the secure enclave (basically a DRM) they’ll get legal protection.
Also. Nobody uses iMessage in the EU. It’s all WhatsApp here. Blue bubbles are an American obsession.