Does this mean what I think it means? That we can finally say goodbye to half a dozen different chat apps and just have one (like Pidgin in the good old days)?
I'm not sure which messenger platforms even need to follow this requirement (e.g. is Signal big enough to have to open up?). Also, there's a few fun open questions like how to uphold end to end encryption or how to exclude spammy messaging services from the network.
Considering how first world governments are fighting against encryption I assume this is just a benefit to them.
Fun fact, I had an interoperable network between leading chat apps Messenger <-> Telegram <-> Viber <-> LINE back in 2017 on top of which we ran a privacy focused dating platform[1]. It was built using bot APIs of these platforms.
In fact, thinking about the exact wording, there might potentially be a loophole that would allow Messenger and WhatsApp to stay incompatible because they're different services by the same provider. But IANAL.
Still I'm skeptical that this draft would see it's light as a law as it is, iMessage is one of the main pillars of Apple ecosystem; Many in U.S. have told me that they must use iPhone as everyone is on iMessage there and I feel Apple will throw everything it has to prevent it being interoperable with WhatsApp in EU which has significant user-base there.