Did Apple remove the "AppleScript handler" option in macOS High Sierra?
Yes, Apple DID remove that feature in 10.13.4. I actually opened a case on this with Apple Support (I even referenced this page) and they got back to me today and informed me that it really has been removed from Messages.  The Apple representative suggested going to http://www.apple.com/feedback/ if I wanted to express my displeasure, but that was all he could do.