Do you test with a proxy? I know I don't, and if I would it'd be the latest version of squid.

Some of those non-conforming middle boxes could be unattended black boxes in the back of a closet somewhere on the 3rd floor. Who knows. Some customers would consider alternatives to your offering because it doesn't work, others would file a support ticket and there becomes an incentive not to use the new and shiny. The broken middle black box vendor can't be bothered and keeps selling broken boxes. Shit like this is why we can't have nice things.