Install MariaDB Server 10.3.12 onto Debian Buster

By use of snapshot.debian.org you can find a .deb package that is version 10.3.12 of mariadb-server. You could download these deb files and install them with apt or dpkg, or by following the instructions on the snapshot web site you could set up your /etc/apt/sources.list to point to a snapshot in time of the Debian archive that corresponds to when that was the latest release.

However, these are strange things to do to a Debian system. By definition the package is old and may not contain important security fixes. You would need a very good reason for doing something like this; generally you should be running the latest supported release either by upstream or by Debian.

Rather than breaking your system in this way it would probably be better to download the source package from upstream and compile it locally, but then there is still the question of whether it contains essential security fixes.