RFC 9122, Section 3.2:
A client MUST send a Host header field (Section 7.2 of [HTTP]) in all HTTP/1.1 request messages. If the target URI includes an authority component, then a client MUST send a field value for Host that is identical to that authority component, excluding any userinfo subcomponent and its "@" delimiter (Section 4.2 of [HTTP]). If the authority component is missing or undefined for the target URI, then a client MUST send a Host header field with an empty field value. A server MUST respond with a 400 (Bad Request) status code to any HTTP/1.1 request message that lacks a Host header field and to any request message that contains more than one Host header field line or a Host header field with an invalid field value.
RFC 9122, Section 3.2:
A client MUST send a Host header field (Section 7.2 of [HTTP]) in all HTTP/1.1 request messages. If the target URI includes an authority component, then a client MUST send a field value for Host that is identical to that authority component, excluding any userinfo subcomponent and its "@" delimiter (Section 4.2 of [HTTP]). If the authority component is missing or undefined for the target URI, then a client MUST send a Host header field with an empty field value. A server MUST respond with a 400 (Bad Request) status code to any HTTP/1.1 request message that lacks a Host header field and to any request message that contains more than one Host header field line or a Host header field with an invalid field value.