Rogers here in Canada. Since last year.
I have the same thing happening. Also with Rogers. I called their tech support, but no help. I tried Google support and they didn't have an answer, but suggested sending in feedback via the Support section on the Pixel.
I did do a factory reset, and this problem started after that. My son also has a Pixel, and has also updated to Oreo. He didn't do a factory reset though, just the OTA update, and his RCS is still working.
I called Rogers. They tried to wipe my profile and then I waited for it to be resupplied. It didn't work.... Google are unaware.
Where did you report that bug? In the app itself ?
I wonder if anyone else on Oreo can try to disable it and re enable it to see if it will go back on