Comment 7 for bug 108296

If you remove the package, the existing databases (and thus /etc/postgresql/<version>/<cluster> are kept, and reinstalling it will work.

If you purge, /etc/postgresql will go away, but then that's what you asked for. The directory is called by pg_createcluster, which is used in the postinst script to create a default 'main' cluster, unless another one exists already or the package is upgraded (not installed freshly).