I dislike how there's no complete open source RCS implementation, and after trying it out a few years ago, I now actively avoid it (I instead use QKSMS on Android)...
But I don't see the problem with the decline link and EU law?
AFAIK, most EU regulations are about tracking and consent in using your information...
In this case, you're already using a Google product (the Messages app), and Google is just (aggressively) nudging you to use extra features that they have shipped in their app. It doesn't follow that Google is definitely going to use more information to track you than it would've done before (though it could be possible, of course)
...of course, I fully agree that this doesn't embody their "respect the user" ethos, but frankly... If you worked on new features for your users, I think it's fair to nudge them to try to make sure that what you worked on will end up benefitting them (of course, a company behaves differently than an individual, and it's not guaranteed that the work done might actually have merit... But that's orthogonal to this discussion)
