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

I've checked all 12 codes against RFC 9110, "HTTP Semantics" (June 2022). Every reason phrase, section number and quote below comes from https://www.rfc-editor.org/rfc/rfc9110.html.

Is RFC 9110 still current? I couldn't fully confirm this. The info page https://www.rfc-editor.org/info/rfc9110 returned HTTP 404, so I couldn't check its "Obsoleted by" list or its errata. Searches turned up nothing that replaces RFC 9110. Its own header says it obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615 and 7694, and parts of 7230.

Heuristic cacheability. Section 15.1 of https://www.rfc-editor.org/rfc/rfc9110.html says codes "defined as heuristically cacheable (e.g., 200, 203, 204, 206, 300, 301, 308, 404, 405, 410, 414, and 501 in this specification)" can be reused with heuristic expiration. It adds that "all other status codes are not heuristically cacheable." So from your list:

• Cacheable: 203, 206, 308, 405, 410, 414 and 501. Each of their own sections also says "A NNN response is heuristically cacheable."
• Not cacheable: 303, 307, 413, 421 and 422. None of their sections says otherwise, so the list and the section texts agree.
• 421: Appendix B.3 notes that 421 "is no longer defined as heuristically cacheable since the response is specific to the connection (not the target resource)."

Name changes. One assumption in the plan was only half right:

• 413 really was renamed. RFC 7231 called it "413 Payload Too Large" (Section 6.5.11, https://www.rfc-editor.org/rfc/rfc7231.html). RFC 9110 calls it "Content Too Large", and "Payload Too Large" no longer appears anywhere in it.
• 422 was not renamed from RFC 7231, because RFC 7231 never defined it (the page contains no "422"). RFC 9110 Appendix B.3 says 422 was "previously defined in Section 11.2 of [WEBDAV]" and moved into RFC 9110. It's now named "Unprocessable Content". I believe WebDAV called it "Unprocessable Entity", but I didn't open that document to check.
• 421 came from RFC 7540, and 308 from RFC 7538.

About the quotes. The page text wraps lines at fixed points, so the quotes join the wrapped lines with single spaces. I picked sentences with no hyphenated line breaks and no bracketed reference links.

{"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":"The 206 (Partial Content) status code indicates that the server is successfully fulfilling a range request for the target resource by transferring one or more parts of the selected representation.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"303","reason":"See Other","section":"15.4.4","heuristicallyCacheable":false,"quote":"This status code is applicable to any HTTP method.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"307","reason":"Temporary Redirect","section":"15.4.8","heuristicallyCacheable":false,"quote":"Since the redirection can change over time, the client ought to continue using the original target URI for future requests.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"308","reason":"Permanent Redirect","section":"15.4.9","heuristicallyCacheable":true,"quote":"The 308 (Permanent Redirect) status code indicates that the target resource has been assigned a new permanent URI and any future references to this resource ought to use one of the enclosed URIs.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"405","reason":"Method Not Allowed","section":"15.5.6","heuristicallyCacheable":true,"quote":"The 405 (Method Not Allowed) status code indicates that the method received in the request-line is known by the origin server but not supported by the target resource.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"410","reason":"Gone","section":"15.5.11","heuristicallyCacheable":true,"quote":"The 410 (Gone) status code indicates that access to the target resource is no longer available at the origin server and that this condition is likely to be permanent.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"413","reason":"Content Too Large","section":"15.5.14","heuristicallyCacheable":false,"quote":"The 413 (Content Too Large) status code indicates that the server is refusing to process a request because the request content is larger than the server is willing or able to process.","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":"This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"}
]}