In theory we could almost make it automatic, right? That is, have a github bot that generates a PR that extracts all exported function names, abstract types and abstract docstrings into an abstract header package, and rewires the old package to require and import and extend and reexport functions from the abstract header package.
In theory, the creation of lightweight AbstractFoo / HeaderFoo packages from some Foo package should not requite a lot of human thought or intervention (corner case: functions and types declared and exported by macro that takes environment into account). If this could be done with close to zero work, and if these lightweight interface packages could be enforced to stay lightweight dependencies, then I think this would get a lot more traction.