Yep I'm running the latest version of Google messages on my factory unlocked pixel with AT&T, and it's working flawlessly, including RCS. I haven't noticed any AT&T interference at all.... it's using jibe through Google and I just update it through the playstore. It seems like when the carriers get involved in the software on the carrier branded phones the problems start... either they disable stock apps to try and get you to pay them for the same thing, or they want to meddle with the apps to "improve" them, mainly it seems to get more personal data out of you, and get you to rely more on their particular "flavor" of that app, to keep you buying their carrier branded/controlled phones. Plus the bloatware
That's why I stopped buying carrier branded phones many, many years ago