As an update to an older article I once posted, it is now much simpler to enforce a strict password policy on Horde through Plesk.
In 11.5.30 plesk and higher, first make sure you’re current on all microupdates
/usr/local/psa/admin/bin/autoinstaller --select-product-id plesk --select-release-current --reinstall-patch --install-component base
and from there all that is required is to change the “Security Policy” in the Plesk control panel and it will impact Horde’s password functions.
Home> Tools & Settings> Security Policy
Just in case anyone out there is following my old article and noticing that the changes they are making are not being applied, it is because Plesk’s implementation of Horde is now using Plesk’s password policy directly.