http://download.owncloud.org/download/repositories/stable/xUbuntu_16.04/ ships now
owncloud-files 10.4.0-1+1.1. This version requires PHP 7.1. As PHP 7.1 is neither available in Owncloud nor Ubuntu repositories, the installation is broken.
I figured this out, after unattended-upgrades did its magic. Now owncloud greets me with
This version of ownCloud requires at least PHP 7.1.0 You are currently running PHP 7.0.33-0ubuntu0.16.04.12. Please update your PHP version