Is this something capping provided?

I had this during legion and assumed it was something capping was providing, with BFA it's no longer present and I can't think of any other addons I was running that would have added that.

It was provided. However it is no longer necessary as the arena frames provide this information.

Not a big fan of the arena frames. That info was compact and concise, while the arena frames are the direct opposite. Can this feature be restored?

It's not a matter of simply restoring it, it would need re-written for 8.0, which I have no interest in. Good unit frame addons tend to support arena frames and thus you can tweak them as you wish.

I use suf, which supports arena frames but afaik cannot display differently based on BG and Arena. If I rewrite and issue a PR would you accept it?

I also use SUF and I am happy with using the same layout for arena & BG. I cannot guarantee I'd accept a PR, it depends on the quality. I'm not really interested in a hackjob that simply makes it work. The best chance of getting it accepted would be creating a "proper" arena frame hardcoded to arena1 and arena2.