They were having issues getting it to work on older devices and the iPad. I expect it will come back for iPhone 7 and later only.
in the materials released along with Xs / Xr launch, on apple's site's pages it is confirming that Group Facetime will be coming "in the Fall" (i presume this to be 2018 Fall...)
that's the rumor. it's also possible that dropping it in the beta showed them that it was popular enough that there are risks of server overload. i still remember the crashing of iCloud when it was launched with the new iOS on release day. last thing they want is a repeat of that. even just iOS and a new device on the same day is bad news, thus why they are releasing iOS 12 4 days before the product launch. get past the initial big wave of activations and then add the nifty new trick
I checked and no iOS was ever release the same day as the iPhone, always a few days before.
In the early betas only audio group FaceTime was available for older devices, and not on the iPad at all.
I wouldn't be surprised if it becomes vaporware...