Death: same way it handles a program with an open socket and data arriving and unread and the program dies. It's just part of the overall resource set of the process and has to be cleaned up when the process goes away.
The do_exit() function in kernel/exit.c is responsible for general cleanup on a process [1]. Whether a process dies gracefully or abruptly, the kernel calls do_exit() to clean up all the resources owned by the process, like opened files or acquired locks. I would imagine the io_uring related stuff is cleaned up there as well.
I don't know the answer but I would assume if you have submitted an operation to the kernel, you should assume it's in an indeterminate state until you get the result. If the program dies then the call may complete or not.
