This is the HTTP/1.1 standard: https://datatracker.ietf.org/doc/html/rfc2616
Then there are also HTTP/2 and HTTP/3.
That's just HTTP, curl supports 27 other protocols.
HTTP/1.1 - June 1999
It's not like the standard changed since curl was created
HTTP/1.1 - June 1999
It's not like the standard changed since curl was created