Igor, we've been running MariaDB 10.2.8 on several Plesk 12.5.30 live servers for months now, including one Onyx instance and we haven't noticed any issues. When you say not supported, is it because it hasn't been tested by Plesk, or because it's been tested and it's not working?
It was not tested because of no one OS vendors ships MariaDB 10.2 at the moment. I mean vendors of OSes, which are supported by Plesk. Even Debian 9 ships MariaDB 10.1