No, that's rather easy for up-to-date browsers. The real reason is that dumb middle proxies might cache/mangle HTTP/2 requests to the point that it simply fails hard, but encryption makes those concerns go away. Yes, there are proxies that can decrypt TLS, but due to how negotiation of HTTP/2 works in TLS (ALPN), dumb proxies will simply not send the signals that it's HTTP/2 capable.