> I have used select and poll before and sort of skipped to io_uring for a recent project, which also doesn't do too well with multiple threads (you have to use multiple rings and do everything else yourself

Wouldn't that be on purpose ? Coordination requires more cpu cycles and so cuts on max performance

> Why was it not ported to Linux?

We've been asking since epoll got introduced.

But here is some context: https://lwn.net/Articles/431297/