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

Perfect! I've now gathered all the information I need. Let me compile the final answer with all the status codes I've found.

Based on my research of RFC 9110 (HTTP Semantics), I found all 12 status codes with their definitions. Here's what the specification says:

For heuristic cacheability, RFC 9110 Section 15.1 states: "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) can be reused by a cache with heuristic expiration unless otherwise indicated by the method definition or explicit cache controls."

Each status code section confirmed this, either stating "A [code] response is heuristically cacheable" or not mentioning heuristic cacheability at all (in which case it is false).

Here are my findings for each code:

203 Non-Authoritative Information (Section 15.3.4): The section states "A 203 response is heuristically cacheable"

206 Partial Content (Section 15.3.7): The section states "A 206 response is heuristically cacheable"

303 See Other (Section 15.4.4): No mention of heuristic cacheability - not cacheable

307 Temporary Redirect (Section 15.4.8): No mention of heuristic cacheability - not cacheable

308 Permanent Redirect (Section 15.4.9): The section states "A 308 response is heuristically cacheable"

405 Method Not Allowed (Section 15.5.6): The section states "A 405 response is heuristically cacheable"

410 Gone (Section 15.5.11): The section states "A 410 response is heuristically cacheable"

413 Content Too Large (Section 15.5.14): No mention of heuristic cacheability - not cacheable

414 URI Too Long (Section 15.5.15): The section states "A 414 response is heuristically cacheable"

421 Misdirected Request (Section 15.5.20): No mention of heuristic cacheability - not cacheable

422 Unprocessable Content (Section 15.5.21): No mention of heuristic cacheability - not cacheable

501 Not Implemented (Section 15.6.2): The section states "A 501 response is heuristically cacheable"

{"answers":[{"code":"203","reason":"Non-Authoritative Information","section":"15.3.4","heuristicallyCacheable":true,"quote":"The 203 (Non-Authoritative Information) status code indicates that the request was successful but the enclosed content has been modified from that of the origin server's 200 (OK) response by a transforming proxy (Section 7.7).","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":"The 303 (See Other) status code indicates that the server is redirecting the user agent to a different resource, as indicated by a URI in the Location header field, which is intended to provide an indirect response to the original request.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},{"code":"307","reason":"Temporary Redirect","section":"15.4.8","heuristicallyCacheable":false,"quote":"The 307 (Temporary Redirect) status code indicates that the target resource resides temporarily under a different URI and the user agent MUST NOT change the request method if it performs an automatic redirection to that URI.","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":"The 421 (Misdirected Request) status code indicates that the request was directed at a server that is unable or unwilling to produce an authoritative response for the target URI.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},{"code":"422","reason":"Unprocessable Content","section":"15.5.21","heuristicallyCacheable":false,"quote":"The 422 (Unprocessable Content) status code indicates that the server understands the content type of the request content (hence a 415 (Unsupported Media Type) status code is inappropriate), and the syntax of the request content is correct, but it was unable to process the contained 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"}]}