[vacri]

The 400-block is 69% free, after 28 years of development. If we're ever in the position where we need to free up just one more code, then we're already consuming beyond our capacity and will need to sort out an alternative anyway.

[vidarh]

Exactly. If we run out of 400-codes without an alternative having been set out, such as e.g. designating one or more of the remaining 400 codes to specify extended formats for, somebody is not doing their jobs.

E.g. once (if ever) we're down to 10 or so free ones, assign them as broad categories and specify that the response text needs to contain an additional, longer status code (so, say, "499 1234 some text" and "499 4242 some text").

It's not as if tech doesn't have a long history of including extension fields like that, and it still works with the existing grammar