Good news! I wish you would say more specifically when
a feature is released. Based on a conversation I had with +Trey Harris
I've been telling people that this is how it worked for the last two days. I tested it myself and it seemed to be working like this two days ago. But this post suggests, you've just implemented the change.
So it took me some testing to figure out what the change actually is. What I figured out is that although the person didn't get added or notified on a sharing-disable post before this change, the change you've implemented makes that behavior more apparent
because the tool no longer lets you use the @[name] to mention them in post. You just have to type in the whole name, M Sinclair Stevens.
I think the error message terminology is a bit confusing. I think it should say "You can't add people to this discussion because sharing has been disabled on this post."