When configuring Apache as a reverse proxy (i.e. using ProxyPass or RewriteRule [P]), does it ever pipeline the requests to origin servers, or are they always sent non-pipelined?
This is important to know if an origin server doesn't handle pipelined requests properly.
No, looking at the source code for mod_proxy (proxy_http_handler() in [modules/proxy/mod_proxy_http.c][1]), the request is sent to the origin server and the same function blocks waiting for a response, before releasing the connection back to the worker pool, so such connections can never be pipelined.
This has the consequence that origin (backend) servers protected by an Apache reverse proxy don't need to be able to handle HTTP pipelined requests.