I have actually had the same issue. I have a few extensions where I had to re-implement KeyboardNavigatable in order to use it in one of my common components.
We probably wouldn't be against a PR that moves the file.
However I'm not sure how to handle the backward-compatibility on the namespace change, I'm not sure we had to do this before. We'd need to make sure the component can still be used via the old namespace (that part should work automatically), but we also need to make sure the typescript typings still resolve for both the old and new namespaces, until the eventual v2 release of Flarum.
https://github.com/flarum/core/blob/master/js/src/forum/utils/KeyboardNavigatable.ts