Almost any web server can be configured to provide a generic response to a specific request, for example by ignoring some or all of the url path, and in practice I would bet that a majority of actual instances do this for at least some sets of requests. (To confirm my theory in 20 seconds I checked if https://news.ycombinator.com/user?id=nonexistent_user_1620 returns a 404 or a 200 - it's the latter.)

It's silly to pretend that the use of a 404 in this type of circumstance is either clearcut in the standards or ubiquitous in practice.