Zero-copy network transmission with io_uring

It depends on what you mean by "the data has been sent." There are multiple possible definitions of that phrase, in chronological order:

0. The kernel has validated the operation (i.e. checked the arguments), and is preparing to push the data we care about into the TCP send queue, but it has not actually done that yet (i.e. it might still be sitting in a userspace buffer). This is probably not what you meant, but I include it for completeness.
1. The data we care about is in the TCP send queue and is enqueued for sending, but due to Nagle's algorithm and/or network congestion, we can't send it yet. Under zero-copy networking, this state is (or should be, anyway) functionally equivalent to state 0, and the application cannot safely free the buffer until it gets a completion notification. Time since previous state: Probably no more than a few hundred microseconds, possibly a lot less in a well-optimized implementation, especially if the data is small or we are async/zero-copy. Might be slower if the data is huge and we have to copy it.
2. The data we care about is "next in line," i.e. it will be transmitted in the next TCP segment, and we are not waiting on any ACKs. Time since previous: Could take multiple seconds if the network is badly congested, or rarely more than that. We might never reach this state, if the network drops altogether or we receive a RST packet. In a well-performing network, tens or hundreds of milliseconds would be typical depending on ping. Or this could be instant, if the send queue was already empty.
3. The data we care about has been packed into one or (rarely) more IP datagrams, and those datagrams have been sent. IP is an unreliable, connectionless protocol, so at the IP layer, sending is an event, not a process. This probably takes no more than a few milliseconds, but I'm not very familiar with this sort of low-level hardware timing, so I might be completely wrong there.
4. The data we care about has been ACKed. At this point, we can be somewhat confident that a well-behaved receiving application on the peer will eventually get a copy of the data, assuming it does not crash or abort before then. Time since previous: At least one ping round-trip, possibly forever if the network drops before we receive an ACK.
5. There has been some sort of application-level acknowledgement of the data we care about, such as an HTTP response code. This may or may not happen at all depending on the protocol and the server/client roles, and the kernel is obviously not in a position to figure that out anyway, so this is a non-starter.

You probably don't mean 0 or 1, because 1 is (I think) when regular old send(2) returns (and 0 is pretty much the exact opposite of "the data has been sent"). But even getting to (2) is potentially multiple seconds and might never happen at all (in which case, I assume you just fail with EIO or something?). If you want that to perform well, you had better not be doing one-thread-per-operation, or you will either spawn way too many threads, or take far too long to accomplish anything. Both of those are bad, so now we're in the realm of async networking and io_uring, or at least the realm of thread pools and epoll, so no matter what, you're going to be doing some sort of async, event-driven programming. There's no way for the kernel to provide the illusion of synchronous networking short of actually doing things synchronously, and that just doesn't scale in any kind of reasonable way. I dunno, maybe your language/OS can fake it using async/await and such? But that's still event-driven programming, it's just dressed up to look like it's synchronous (and the kernel has no part in this chicanery anyway).

Even getting to (4) is not really enough to have any assurances, because you have to treat every element of the system as unreliable, including the remote host. The peer could still crash after (4) and never see your data (because it was sitting in the receive queue). Until you see (5), you simply cannot know whether the peer received or will receive the data. And the kernel can't figure out whether (5) has happened for you, because (5) is application-specific.