Single on Twelve HTTP status codes, by the book

All 12 codes are now checked against RFC 9110, and I'm putting the answer together.

The current HTTP Semantics specification is RFC 9110 (https://www.rfc-editor.org/rfc/rfc9110.html). Every quote, section number and reason phrase below comes from that page.

Which codes are heuristically cacheable: Section 15.1 lists the codes RFC 9110 treats as heuristically cacheable: "200, 203, 204, 206, 300, 301, 308, 404, 405, 410, 414, and 501 in this specification". It also says "all other status codes are not heuristically cacheable" (https://www.rfc-editor.org/rfc/rfc9110.html).

• Heuristically cacheable (true): 203, 206, 308, 405, 410, 414 and 501. Each one's own section says "A NNN response is heuristically cacheable".
• Not heuristically cacheable (false): 303, 307, 413, 421 and 422. None of their sections includes that sentence.

Two exceptions in the wording:

• 206: its section leaves out "by the method definition". It reads "unless otherwise indicated by explicit cache controls".
• 413 and 422: RFC 9110 renamed them "Content Too Large" and "Unprocessable Content". Older RFCs and many libraries still use "Payload Too Large" and "Unprocessable Entity". The reasons below use the current names.

In the quotes, the page's line breaks are replaced with single spaces.

{"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 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 (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"}
]}