This is formalizing existing practice, not introducing a new concept.
[0] https://en.wikipedia.org/wiki/List_of_HTTP_status_codes#420
[0] https://en.wikipedia.org/wiki/List_of_HTTP_status_codes#420
And let's be honest, it's a great name when the lowest free options are 419 or 420.
Some may roll their eyes at it, but it would certainly be a memorable status code.
And yes, that would be way more useful.
...that sounds like an example use case for the proposed status code.
420: Requester Impaired - Infinite loop
It would allow you to signal to the requester that while their individual responses are fine, their session behavior is incoherent.