Open-Xchange is still available in the App Center, but currently limited to UCS 4.1 and UCS 4.2. The current version of Open-Xchange only run on Debian Jessie and therefore only up to UCS 4.2.
UCS 4.3 is based on Debian Stretch and OX App Suite 7.8.4 does not support it. The next version 7.10 will support Debian Stretch and therefore also UCS 4.3. According to our information, OX App Suite 7.10 will be available in the second quarter 2018.