Restricted environments in larger corporations can do a full mitm proxy

Eh, if you're dealing with corporate network proxies all bets are already off. They keep blocking connections for the most random reasons until everyone is opening ssh tunnels just to get work done. Websockets are a standard feature of the web, if you cut off your ears don't complain about loss of hearing. Unless, you're explicitly targeting such corporations as clients, in which case - my condolences.

It's not a very good man-in-the-middle if it can't handle a ubiquitous protocol from 2011 based on http/1.1. More like an incompetent bureaucrat in the middle.