505 HTTP Version Not Supported
Meaning
The server does not support, or refuses to support, the HTTP protocol version used in the request. This implies the server cannot or will not use the protocol version to fulfill the request.
When to Use
When a client uses a newer or incompatible HTTP version that the server cannot handle. For instance, a server that only supports HTTP/1.1 might respond with 505 if it receives a request marked as HTTP/2 (and it hasn't upgraded or isn't capable).
Example
If a client mistakenly sends an HTTP/2.0 request to a server that only understands HTTP/1.1 (without negotiating via upgrade), the server could respond with 505 HTTP Version Not Supported indicating it can't process that version.
When Not to Use
Do not use if the issue is not with the protocol version. If content is fine but something else fails, use the relevant status. 505 is rarely used because version negotiation usually happens differently (e.g., ALPN for HTTP/2).
Source
RFC 9110