tldr; both HTTP/1.1 and HTTP/2 header names are case-insensitive BUT HTTP/2 enforces lowercase header names.

HTTP/1.1

According to RFC 7230, section 3.2:

Each header field consists of a case-insensitive field namefollowed by a colon (":"), optional leading whitespace, the fieldvalue, and optional trailing whitespace.

HTTP/2.0

Quoting RFC 7540, section 8.1.2:

Just as in HTTP/1.x, header field names are strings of ASCII
characters that are compared in a case-insensitive fashion.

...but then:

However, header field names MUST be converted to lowercase prior to their encoding in HTTP/2. A request or response containing uppercase header field names MUST be treated as malformed.