4×× CLIENT ERROR
404 NOT FOUND
The origin server did not find a current representation for the target resource or is not willing to disclose that one exists.
A 404 status code does not indicate whether this lack of representation is temporary or permanent; the 410 Gone status code is preferred over 404 if the origin server knows, presumably through some configurable means, that the condition is likely to be permanent.
A 404 response is cacheable by default; i.e., unless otherwise indicated by the method definition or explicit cache controls.
Calculating Heuristic Freshness RFC7234 Section 4.2.2
404 NOT FOUND Source: RFC7231 6.5.4
404 CODE REFERENCES
Rails HTTP Status Symbol
Go HTTP Status Constant
Symfony HTTP Status Constant
Python2 HTTP Status Constant
Python3+ HTTP Status Constant
Python3.5+ HTTP Status Constant