You can use userspace Coroutines/fibers implementations to wire in async io_uring into existing synchronous code and maintain the facade of the code still being synchronous
How easy/feasible this is depends on the language.
In C++, Rust, Zig, Java (Loom fibers), and Kotlin I know for a fact it's doable
Other languages I'm not sure what the experience is like
But in this case are you really reaping any benefit of io_uring, if your existing code is still synchronous (except your io_uring layer)?
It seems to me to see any substantial benefit, you'd have to propagate async throughout your codebase, which would require rewriting your existing code to use some async primitive. I guess something like Java Loom does help you mask this, however.
How easy/feasible this is depends on the language.
In C++, Rust, Zig, Java (Loom fibers), and Kotlin I know for a fact it's doable
Other languages I'm not sure what the experience is like