The RFC explains.

The other side - the server - will be stuck on `send(sock_fd, more_bytes...)`. If it's the 90s and your FTP server is single-threaded, that means the server will appear completely stuck. This won't resolve for several minutes, or possibly forever if the server-side TCP stack is buggy or lacks timeouts.

The client's connection, even after the process is gone, will still be alive in the kernel. It will be in FIN-WAIT-2, waiting for the server to send its FIN. The remote sender will be stuck waiting for the zero-window state to pass, sending probes to figure out when the receive window opens (which will never happen). Things will be stuck in this state until one of the sides times out, which could be several minutes. Or if the implementations are buggy, it could be permanent.

From RFC 2525, 2.17

      Failure to reset the connection can lead to permanently hung
      connections, in which the remote endpoint takes no further action
      to tear down the connection because it is waiting on the local TCP
      to first take some action.  This is particularly the case if the
      local TCP also allows the advertised window to go to zero, and
      fails to tear down the connection when the remote TCP engages in
      "persist" probes (see example below).

In some protocols, end-of-file has semantic meaning that all data has been transferred, and TCP is set up such that you should be able to rely on that - if you can't rely on that difference, it is a bug in a TCP stack along the way.

FIN also has a sequence number, so you can wait to ACK it until you get the corresponding data if it is dropped or out of order.

TCP RST says the other side won't be resending if not ACKed, it is reset. Further, the downloading client usually cannot even read any packets in the receive window either once an RST has been received - that might be hundreds of KB of missed data.

RST and FIN are very semantically meaningfully different.

Reading the post, if gunicorn is e.g. sending a 404 after seeing a POST to a path it doesn't know about before reading the body, the client will never get the 404 because gunicorn hasn't read the message body.

This case is partly why "Expect: 100-continue" exists, so it will be properly handled, even if it does introduce an extra round-trip lag in the POST.

It might be dangerous to have your protocol rely on a piece of TCP that is often incorrectly implemented.