You could make it a reasonable binary if you define unsafe as "has known bugs that could lead to deaths", which is what the top comment was doing. Not sure if any already-in-use planes satisfy that criterion though.
But when the bug is known to be hit only is astronomically slim scenarios, they don’t rework the entire stack to eliminate it because, well perhaps it is more likely to cause more harm than good?
"unaccepted by users" is circular in this particular discussion, because it started with trying to tease out whether a rare* safety* risk was "acceptable."
* All these debatable words tell me GuB-42's comment is taking the right approach.
I wouldn't include that since it wouldn't be useful. In any case it clearly wasn't included in whatever definition the top commenter intended.