Well, that traditon ended something like 20 years ago 
For long time now everyone is trying to do 404 page as helpful as possible to offer to an user something else.
404-page of Discourse is not the worst, though. There is search plus lists of popular and latest. In perfect world it should offer some related content (that’s why we have categories and tags) because opposite to general believe 404 is almost never user’s error. It comes from site where is no proper 301 redirections.
Actually… getting 404 is always a mark of lazyness from site
But… is this something that the team should fix asap? No, but there is some bigger questionmarks with deleted content and easiness of creation of redirections. But these aren’t too high on major tasklist, I reckon. But I could guess it would be something that hosted big fishes would like.