There's multiple issues. The biggest ones that I can recall, off the top of my head:

First off, do notation desugars to closures. But Rust has three different kinds of closures. How does that work?

Furthermore, Rust has imperative control structures. How does all of this interact with each other? Are you now no longer allowed to use those constructs inside of do notation? That feels inconsistent.

I don't know of any languages that have something like do notation but don't have everything boxed. It makes everything easier, but that's not generally acceptable in Rust. For example, we can use async/await with no dynamic allocation. Could we with do notation?

"Open question" doesn't mean "impossible", mind you. But nobody has ever come up with a design. In the meantime, we have users to support...