My repo is complex - I only want to share a portion

Scenario: Author keeps several papers related to an ongoing or long-running project in a single repo, and wants to isolate code for tidy submission alongside an “offshoot paper”.

In this case, it has been suggested to use the git filter-branch approach, which is essentially a way to split off a subset of a repo into a new repo (e.g. a subdirectory paper-1). It will conveniently inherit the commit history as well (but see above if that is not desired). See guide at https://help.github.com/en/articles/splitting-a-subfolder-out-into-a-new-repository.

(Thanks to @MichaelChirico for suggesting)