I think it's more clear to say that headers are usually specified separate from the protocols these days, so there are new headers introduced after HTTP/1.1, bu

I think it's more clear to say that headers are usually specified separate from the protocols these days, so there are new headers introduced after HTTP/1.1, but they are not usually introduced in the core HTTP spec but rather in separate RFCs that usually apply to HTTP/1.1, HTTP/2 and HTTP/3. The only new header in the HTTP/2 RFC is specifically for HTTP/2 upgrades from HTTP/1.1: https://tools.ietf.org/html/rfc7540#section-3.2.1 https://tools.ietf.org/html/rfc7540#section-3.2.1