See https://lists.w3.org/Archives/Public/ietf-http-wg/2015JulSep/0193.html:
"Alternative Services MUST NOT be advertised for a protocol that is not designed to carry the scheme. In particular, HTTP/1.1 over TLS cannot carry safely requests for http resources."
...which refers to the :scheme pseudo header field in HTTP/2 (http://greenbytes.de/tech/webdav/rfc7540.html#HttpRequest).
As far as I recall the intention of the statement above is to avoid that when alt-svc is used to move http traffic to a TLSsy port such as 443, the alternative server gets confused about whether it's serving HTTP or HTTPS.
The clause seems to be less relevant when alt-svc is used to load-balance HTTP/1.1 http_s_ traffic.
See https://lists.w3.org/Archives/Public/ietf-http-wg/2015JulSep/0193.html:
"Alternative Services MUST NOT be advertised for a protocol that is not designed to carry the scheme. In particular, HTTP/1.1 over TLS cannot carry safely requests for http resources."
...which refers to the :scheme pseudo header field in HTTP/2 (http://greenbytes.de/tech/webdav/rfc7540.html#HttpRequest).
As far as I recall the intention of the statement above is to avoid that when alt-svc is used to move http traffic to a TLSsy port such as 443, the alternative server gets confused about whether it's serving HTTP or HTTPS.
The clause seems to be less relevant when alt-svc is used to load-balance HTTP/1.1 http_s_ traffic.