@ahartikainen, your comment helped me find one instance of a importFrom of a package I no longer wanted as a dependency, and made me stumble across this post I somehow missed: How to update a package that has StanHeaders and rstan in its LinkingTo. (I had been trying to avoid updating rstan while writing these papers, but I guess that ship sailed.) Thank you!
It still doesn’t pass on Travis, but it’s closer than it was.