> That said, using a vintage technology has some downsides. It was never updated to support WebSockets

With widespread browser support for WHATWG streams, it's pretty easy to implement your own WebSockets over long-lived HTTP requests. Basically you just send a byte stream and prepend each message with a header, which can just be a size in many cases.

Advantages over WebSockets:

* No special path in your server layer like you need for WebSocket.

* Backpressure

* You get to take advantage of HTTP/2/3 improvements for free

* Lower framing overhead

Unfortunately AFAIK it's still not supported to still be streaming your request body while receiving the response, so you need a pair of requests for full bidirectional streaming.