Plan-execute-verify on Twelve HTTP status codes, by the book

The sources I opened show no newer replacement for RFC 9110, which is the current HTTP Semantics specification (STD 97). Every code below is defined in section 15 of https://www.rfc-editor.org/rfc/rfc9110.html. I read each section's text on that page.

Cacheability rule. The page says "Responses with status codes that are defined as heuristically cacheable (e.g., 200, 203, 204, 206, 300, 301, 308, 404, 405, 410, 414, and 501 in this specification)". Of the 12 codes you asked about, that list covers 203, 206, 308, 405, 410, 414 and 501. Each of those sections also says "A NNN response is heuristically cacheable". The sections for 303, 307, 413, 421 and 422 say nothing about heuristic cacheability, and the same passage states "all other status codes are not heuristically cacheable", so I report them as false.

Reason phrases. RFC 9110 renamed some phrases compared with the older RFC 7231 wording. The current phrases are 413 "Content Too Large" and 422 "Unprocessable Content". I did not open RFC 7231's text for this comparison, so the older names ("Payload Too Large", "Unprocessable Entity") come from my memory rather than from a page I read.

Plan deviation. The plan suggested checking the rfc-editor.org info page for "Obsoleted by". That URL returned a 404, so I used a search instead. The search results gave no sign of a successor.

{"answers":[
{"code":"203","reason":"Non-Authoritative Information","section":"15.3.4","heuristicallyCacheable":true,"quote":"This status code allows the proxy to notify recipients when a transformation has been applied, since that knowledge might impact later decisions regarding the content.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"206","reason":"Partial Content","section":"15.3.7","heuristicallyCacheable":true,"quote":"A client MUST inspect a 206 response's Content-Type and Content-Range field(s) to determine what parts are enclosed and whether additional requests are needed.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"303","reason":"See Other","section":"15.4.4","heuristicallyCacheable":false,"quote":"Except for responses to a HEAD request, the representation of a 303 response ought to contain a short hypertext note with a hyperlink to the same URI reference provided in the Location header field.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"307","reason":"Temporary Redirect","section":"15.4.8","heuristicallyCacheable":false,"quote":"The user agent MAY use the Location field value for automatic redirection.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"308","reason":"Permanent Redirect","section":"15.4.9","heuristicallyCacheable":true,"quote":"The server SHOULD generate a Location header field in the response containing a preferred URI reference for the new permanent URI.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"405","reason":"Method Not Allowed","section":"15.5.6","heuristicallyCacheable":true,"quote":"The origin server MUST generate an Allow header field in a 405 response containing a list of the target resource's currently supported methods.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"410","reason":"Gone","section":"15.5.11","heuristicallyCacheable":true,"quote":"The 410 response is primarily intended to assist the task of web maintenance by notifying the recipient that the resource is intentionally unavailable and that the server owners desire that remote links to that resource be removed.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"413","reason":"Content Too Large","section":"15.5.14","heuristicallyCacheable":false,"quote":"If the condition is temporary, the server SHOULD generate a Retry-After header field to indicate that it is temporary and after what time the client MAY try again.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"414","reason":"URI Too Long","section":"15.5.15","heuristicallyCacheable":true,"quote":"The 414 (URI Too Long) status code indicates that the server is refusing to service the request because the target URI is longer than the server is willing to interpret.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"421","reason":"Misdirected Request","section":"15.5.20","heuristicallyCacheable":false,"quote":"A proxy MUST NOT generate a 421 response.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"422","reason":"Unprocessable Content","section":"15.5.21","heuristicallyCacheable":false,"quote":"For example, this status code can be sent if an XML request content contains well-formed (i.e., syntactically correct), but semantically erroneous XML instructions.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"501","reason":"Not Implemented","section":"15.6.2","heuristicallyCacheable":true,"quote":"The 501 (Not Implemented) status code indicates that the server does not support the functionality required to fulfill the request.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"}
]}

Note: the 421 quote ("A proxy MUST NOT generate a 421 response.") is only 8 words but meets the six-word minimum.