Fortunately, all it takes to install the univention-custom-ldap-acls and univention-custom-ldap-acls-schema packages is to use 2to3 to convert the embedded script from Python 2.x to Python 3.x (for reference package management - Easily unpack DEB, edit postinst, and repack DEB - Unix & Linux Stack Exchange).
I was then able to see the DNs where authorization has been configured and remove the ACL references in them. Afterwards I uninstalled the packages again.
The only remains are
6299e393 UNKNOWN attributeDescription "UNIVENTIONCUSTOMACLREFERENCEGROUPMODIFY" inserted.
I followed Remove LDAP schema extensions and Problem: After a ldap schema was removed, there are still some references in your ldap but the generated backup.ldif does not contain any reference to the aforementioned attribute description.
Any hints?