Duplex streams are not really a HTTP/2-only feature. You can do the same bidirectional streaming with HTTP/1.1 too. The flow is always: 1. The client sends a header set. 2. It can then start to stream data in the form of an unlimited-length byte-stream to the server. 3. The server starts to send a header set back to the client. 4. The server can then start to stream data in the form of an unlimited-length byte-stream to the client.

There is not even a fixed order between 2) and 3). The server can start sending headers or body data before the client sent any body byte.

What is correct is that a lot of servers and clients (including javascript in browsers) don't support this and make stricter assumptions regarding how HTTP requests are used - e.g. that the request bytes are fully sent before the response happens. I think ReadableStream/WritableStream APIs on browsers were supposed to change that, but I haven't followed the progress in the last few years.

NGINX falls into the same category. It's HTTP/2 support (and gRPC support) had been built with a very limited use-case in mind. That's also why various CDNs and service meshes use different kinds of HTTP proxies - so that various streaming workloads don't break in case way the protocol is used is not strictly request->response.