Deleting a process disconnects Emacs immediately from the subprocess. Processes are deleted automatically after they terminate, but not necessarily right away. You can delete a process explicitly at any time. If you explicitly delete a terminated process before it is deleted automatically, no harm results. Deleting a running process sends a signal to terminate it (and its child processes, if any), and calls the process sentinel. See Sentinels: Detecting Process Status Changes.
When a process is deleted, the process object itself continues to exist as long as other Lisp objects point to it. All the Lisp primitives that work on process objects accept deleted processes, but those that do I/O or send signals will report an error. The process mark continues to point to the same place as before, usually into a buffer where output from the process was being inserted.
This variable controls automatic deletion of processes that have
terminated (due to calling exit or to a signal). If it is
nil, then they continue to exist until the user runs
list-processes. Otherwise, they are deleted immediately after
they exit.
This function deletes a process, killing it with a SIGKILL
signal if the process was running a program. The argument may be a
process, the name of a process, a buffer, or the name of a buffer. (A
buffer or buffer-name stands for the process that
get-buffer-process returns, and a missing or nil
process means that the current buffer’s process should be
killed.) Calling delete-process on a running process
terminates it, updates the process status, and runs the sentinel
immediately. If the process has already terminated, calling
delete-process has no effect on its status, or on the running
of its sentinel (which will happen sooner or later).
If the process object represents a network, serial, or pipe
connection, its status changes to closed; otherwise, it changes
to signal, unless the process already exited. See process-status.
(delete-process "*shell*")
⇒ nil