[3np]

I don't see a fundamental problem here - obviously the API has to be made public and free for others to implement inside the EU.

Allow users to bring their own client. Depending on particulars in resulting regulation, could also mean federation (where S2S means passing on encrypted messages, the content of which is of course encrypted).

Maybe not everyone here are aware that WhatsApp, FB Messenger and Google Talk/Hangouts/whatever were all speaking XMPP before they eventually went closed. There was a time when you could connect to all of them from the same client speaking the same protocol, and talk to people on different servers. WhatsApp's server side started as a fork of ejabberd.

The most straightforward way (assuming non-malicious compliance, which TBF may be far-fetched) for this would probably be XMPP with a well-defined set of extensions. This would not require compromising user security, nor would it require SSO.

[zaik]

WhatsApp still uses XMPP, but has never federated with other servers.

[MattJ100]

I think it's a stretch to make that claim unless you have evidence to back it up. The last time I saw WhatsApp protocol stuff it was certainly recognisable as something derived from XMPP, but very far from simply a non-federated standard XMPP service.