I'm on LineageOS, rooted, and RCS is working for me. Is the block tied to the device safety attestation?
My device (somehow?) passes safety check, and I've had no problems with my banking app, Pay, or other annoying apps that insist you don't own your own device.
I lost RCS for most of yesterday on my rooted stock phone. After I realized what was going on I went into Magisk and saw that an update was available for Play Integrity Fix and that solved the issue. What I found most annoying wasn't that it didn't work, but that it broke in a way that seemed like it was working. Both myself and other people saw that we were connected/chatting via RCS and their messages sent just fine, but I didn't receive them. Well, until I applied the update, then I got all of them.
No, I have Magisk which is probably what it is. I'm still used to the r/LineageOS rules where you're not allowed to even mention the existence of Magisk. lol
You can use Simple Play Integrity Checker on IzzyOnDroid (fdroid repo) or the play store. It also checks safetynet.
If gpaywallet works that would indicate 2/3 play integrity if your android is recent I think, so it might be unrelated to the api. 3/3 isn't enforceable yet afaik, too many legitimate devices can't pass it.
Apps can still bring their own root checks, but magiskhide should allow you to pass those.
Unless the app is quite nasty and checks for the mere presence of apps that don't do much without root or xposed, in which case there are also modules that allow you to limit what apps other apps can see being installed (really seeing installed apps should be a permission, but sadly it isn't on most roms yet)