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

I checked all 12 codes against RFC 9110, HTTP Semantics (June 2022), at https://www.rfc-editor.org/rfc/rfc9110.html. It is still the current spec: its header lists the RFCs it obsoletes (2818, 7230, 7231 and others) and has no "Obsoleted by" note. Every value below comes from that one page.

Which codes are heuristically cacheable. Section 15.1 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) can be reused by a cache with heuristic expiration unless otherwise indicated by the method definition or explicit cache controls [CACHING]; all other status codes are not heuristically cacheable." (https://www.rfc-editor.org/rfc/rfc9110.html)

• Of the codes you asked about, 203, 206, 308, 405, 410, 414 and 501 are heuristically cacheable. Each of their own sections also says so, e.g. "A 405 response is heuristically cacheable…".
• 303, 307, 413, 421 and 422 are not. Their sections say nothing about caching, so the "all other status codes" rule applies.
• One small difference: 206's section leaves out "the method definition" and says only "unless otherwise indicated by explicit cache controls".

Section numbers and reason phrases (all from https://www.rfc-editor.org/rfc/rfc9110.html):

• 203 Non-Authoritative Information: §15.3.4
• 206 Partial Content: §15.3.7
• 303 See Other: §15.4.4
• 307 Temporary Redirect: §15.4.8
• 308 Permanent Redirect: §15.4.9
• 405 Method Not Allowed: §15.5.6
• 410 Gone: §15.5.11
• 413 Content Too Large: §15.5.14
• 414 URI Too Long: §15.5.15
• 421 Misdirected Request: §15.5.20
• 422 Unprocessable Content: §15.5.21
• 501 Not Implemented: §15.6.2

Everything matched the expected values in the plan.

Things to be aware of:

• Two names changed in this RFC. 413 is now "Content Too Large" (it used to be "Payload Too Large"), and 422 is now "Unprocessable Content" (it used to be "Unprocessable Entity"). I used the current names.
• Reason phrases are only recommendations. Section 15.1 says they "can be replaced by local equivalents or left out altogether without affecting the protocol."
• Line breaks in quotes. The RFC page wraps its text across lines with extra spaces. In the quotes I joined the lines with single spaces and changed nothing else. None of the quotes I picked contain a hyphen split across a line.

{"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"}
]}