That is something I don’t miss at all from Android after abandoning Android for iOS 3 years ago (Samsung started locking down their phones with the Galaxy S5, decided I wanted no more part of Samsung bloatware and losing the ability to rid myself of it).

Chat heads are a great feature, and of course you can turn them off if you like. iOS doesn't even give you the option to do that.

Chat heads are a terrible feature, and now that you told me I can turn them off I’m going to go do that immediately.

What's your main issue with them? Aren't they just a superior version of the "reply-in-notification" feature of iOS?

The reply-in-notifications go away after I've replied, they don't hang around and cover a portion of the screen all the time. And if I don't want to read them right away, then I've got this thing hanging around on my screen with a angry red number that I don't want to deal with.

They're intrusive, they're obnoxious, they cause performance hiccups on occasion. They're not a superior version of anything.

I had to google it, they look pretty distracting to me, but I'm positive that if they turn out to be very useful Apple will release an API for it.

They haven't in years and I pray every day that it stays that way.