Uninstalling a package usually does not purge configurations (from /etc) and LDAP-entries. While the first cleanup can be done with apt (remove --purge) the second means some deeper digging using univention-ldapsearch. I am not very familiar with the Horde integration and dont know where to look at.
The source of the portal entry can be checked with udm settings/portal_entry list --filter=cn=horde.
/horde is proxied to docker:
root@d:/etc/apache2/sites-enabled# grep -i horde *
000-default.conf: ProxyPass /horde http://127.0.0.1:40000/horde retry=0
000-default.conf: ProxyPassReverse /horde http://127.0.0.1:40000/horde
default-ssl.conf: ProxyPass /horde https://127.0.0.1:40001/horde retry=0
default-ssl.conf: ProxyPassReverse /horde https://127.0.0.1:40001/horde
root@d:/etc/apache2/sites-enabled# netstat -tulpen |grep 4000
tcp6 0 0 :::40000 :::* LISTEN 0 31733 2679/docker-proxy
tcp6 0 0 :::40001 :::* LISTEN 0 31705 2668/docker-proxy