For unstable the plan is to upload MariaDB 10.5 soon, and therefore uploads of MariaDB 10.3 are already discontinued. Since we already have MariaDB 10.4 in Debian experimental, it is not even possible to do any uploads of MariaDB 10.3 because of triggering the NEW queue and version conflicts/downgrades.
A stable update for Buster with latest MariaDB 10.3 is in the works.
