While there really is a trend to overly dumb down things, I'd just like to speculate that the "message no longer exists" part is simply these when someone adapt

While there really is a trend to overly dumb down things, I'd just like to speculate that the "message no longer exists" part is simply these when someone adapted the reasonable error for a delete message, and forgot to actually check if that phrasing work anymore. Small changes creeping up into something stupid is a common thing as well when everyone just does a tiny isolated part.