Since when are they case sensitive? https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/... says otherwise.

It's possible for a server to treat them as case sensitive, but that seems like a bad idea.

+1

HTTP/2, headers are not unique if they only differ by casing, but they must be encoded as lowercase.

   Just as in HTTP/1.x, header field names are strings of ASCII characters that are compared in a case-insensitive fashion. 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 (Section 8.1.2.6).[1]

   Each header field consists of a name followed by a colon (":") and the field value. Field names are case-insensitive.[2]

[1]: https://datatracker.ietf.org/doc/html/rfc7540#section-8.1.2

[2]: https://datatracker.ietf.org/doc/html/rfc2616#section-4.2