Apple didn't explain why it was cutting the feature, but it has been working in previous betas. It's not exactly a simple chat implementation, though. Group FaceTime lets up to 32 people participate in a single audio or video conversation at one time, complete with shifting windows as participants speak up. The last-minute cut (iOS 12 and Mojave are expected in September) suggests Apple wasn't content with how well this many-person technology was working and decided that it was safer to push it back than irk users.
Whatever the reason, it continues a pattern of Apple pushing back software that isn't quite ready for prime time. With that said, a fall release is better than what happened with iOS 11. Users had to wait until late May to get features Apple had announced nearly a year ago, including AirPlay 2 and Messages in iCloud.