The handshake requires sending 8 bytes of body data, without a suitable content-length header. Without the content-length header, http proxies won't pass on those 8 bytes, so handshakes fail.
Hopefully this is addressed in the newest revision.
The latest version is probably the most robust if your ISP likes fiddling with your HTTP though.
I don't think reverse proxies are that big a deal personally, as you as a website owner have control of that part of the system. If the reverse proxy can't deal with it, fix it so it can or use a different one or write your own.
There's certainly ISPs out there that proxy all HTTP through their own cache/proxies and fallover with previous spec websocket. Of course the alternative is just to use HTTPS.