[pornel]

Corporate/country/personal proxies, firewalls and antivirus software don't perform redirects themselves, so they are irrelevant for status 303 support.

Today all clients that send HTTP/1.0 version support more than HTTP/1.0, e.g. HTTP/1.1 Host header is basically mandatory on the web.

HTTP/1.1 compliance requires some harder things like pipelining support, and full HTTP/1.1 caching proxy has a lot of hairy stuff to deal with (strong/weak cache validators, stitching of partial responses, Vary support, etc.).

You can have quite decent HTTP/1.1 implementation that still doesn't deserve to be called full HTTP/1.1.