There are ~32k ephemeral ports. Typical servers have ~32G of memory. It's certainly not hard to imagine a request architecture where a single request can be handled in less than 1MB of per-request memory.

Yes, but you don't need a separate port for every request. Is it reasonable to believe that a single client will have that many conns open?

In this case, they were reverse proxying with an old nginx that only supported HTTP/1.0 for backend connections. So they do need an ephemeral port for each request.

so we've made a few changes since I'd initially done the ephemeral port tuning, the most important being switching to unix domain sockets rather than TCP. with that, we probably no longer need the ephemeral port setting.

Yes. In some circumstances.