The more I think about iOS 11.4 and what features are included in this first beta, the more curious it seems. In particular, I was using the early betas of 11.3. They were... rough. In particular, b1 was one of the most frustrating betas I've used since iOS 5. But I was heavily using two features that were destined to be pulled, Multi-Destination AirPlay 2, iMessages in the Cloud. My experiences with those weren't particularly bad. Never noticed a lost iMessage, and my only gripe with AirPlay 2 was some difficulty presenting the AirPlay destination screen from Overcast and Music, a problem which still hasn't gone away.
Obviously my experiences weren't universal here, and iMessage in the Cloud in particular needs to be bulletproof before it rolls out, but I haven't seen any other significant criticism of the states of these features in the early betas that would have justified pulling them so early in the beta cycle. Also, references to ClassKit were found in the first 11.3 beta, so it seems like at some point that was slated for that release too, and considering 11.3 was launched right after Apple's Education event it would have worked out really well. So what happened here?
Here's my completely uninformed theory. ClassKit was delayed so that Apple's whole school story could kick off in June with the end of the school year and the start of prep for next year. At some point though the decision was made for it to not be a patch release on 11.3, but rather a new point release as 11.4. But 11.4 couldn't launch only with a SDK for a niche set of apps that was only going to be in beta anyway. So iMessage in the Cloud and AirPlay 2 were pulled out, not because they weren't ready, but for marketing reasons. This also follows with the recent reports that Apple is giving engineers more flexibility with ship dates. I'm certain that, however ready or not ready these features were, the engineers working on the team appreciated the ability to spend a few more months working rather than shipping the first version and following up with patches.
Most of this is going to be impossible to verify, but it's fun to speculate that Apple is actually committing to this whole "Give engineers more time to iron things out, rather than rushing initial versions of features" attitude. Although it certainly seems in this case that making the 11.4 release feature-full is giving HomePod buyers even more time without key features of the product.