As someone who receives slacks, I'd also love for this not to be optional. I don't want to talk to someone IRL and have them look at me blankly when I describe something they allegedly wrote to me.
We will definitely make this a feature! Providing awareness on whether it is an AI or human response will be key for trust with colleagues. It is worth noting that messages are approved before being sent so you have total visibility as to what is going out.