The proxy would redirect HTTPS requests to HTTP, and use HTTP 1.1 between the client and the proxy. The bigger issue is HSTS; the proxy can remove the header,

The proxy would redirect HTTPS requests to HTTP, and use HTTP 1.1 between the client and the proxy. The bigger issue is HSTS; the proxy can remove the header, but if the origin comes preloaded in the browser, that won't do, which means the browser would need to be customized to not have or enforce HSTS.