Then all bets are off, and I guess we just can’t HTTP ever again because a proxy can misbehave.
One should adhere to Best Practices since one cannot control every device between the app and the user. Best Practice says “GET has no body. QUERY can have a body. If QUERY fails (405), use POST with the body.” And eventually, enough proxies will behave well enough that at least the HTTP bit of the app has a chance of working.
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.