After reviewing a stretch system I see that I've mischaracterized the issue. Stretch has postgresql-9.6-postgis-2.3 and postgresql-9.6-postgis-2.3-scripts which are analogous to buster's v11 packages, including being recommendations rather than depends. My notes don't say anything about me manually installing these but I must have.
This is a weird situation though, with the postgis extension not being in the postgis package, and the package instead having some utilities only tangentially related to the extension. Perhaps what is needed is a meta package (like postgresql) so someone can install the extension once and have it upgraded across releases. The stuff currently in the postgis package should be moved to an optional postgis-utilities package (recommended by, but not a dependant of the meta package).