I'm so glad someone finally wrote an article about this -- I've been feeling this way for a while. The Away Message served so many vital functions in the synchronous chat modality. We need it more than ever, now that we're in a constant state of synchronous chat availability.

Apple's done this to some extent with iOS 15's set of "Focus" features. The most direct use being that if someone enabled Do Not Disturb (or other Focus mode), others will see their iMessage chat as "$person has notifications silenced". This is a key step to bringing back the Away Message concept, and I hope Apple and others continue expanding upon this.