In postgis LTS update 2.3.1+dfsg-2+deb9u1, the package postgresql-9.6-postgis-2.3-scripts is empty (containing only /usr/share/doc files). The scripts are missing. Without the scripts, I believe it's not possible to create new postgis databases.
To reproduce:
$ sudo -u postgres createdb test
$ sudo -u postgres psql test -c 'create extension postgis'
ERROR: could not open extension control file "/usr/share/postgresql/9.6/extension/postgis.control": No such file or directory