Yes. The Internet-side client and the proxy server can use different protocols and versions that they can negotiate however they like, receive the HTTP request over those, and the proxy can use a separately-negotiated connection to your server to send that request over.

Essentially, the core of standard HTTP requests and responses (method, path, headers, response code) is fairly unchanged since HTTP/1.0, and what's changed is how those requests/responses are encoded and carried.

Yes, you can enable H3 support in the Cloudflare dashboard. We serve... lots... of H3 requests every second on behalf of our customers.

More on our blog https://blog.cloudflare.com/http3-the-past-present-and-futur...