This is a good question and important one to understand to anyone running Ubuntu 20.04 with Virtualmin 6.
The short answer is no, meaning that we will not replace MySQL with MariaDB on existing installs, even if you upgrade to Virtualmin 7 as long as you stay on /vm/6 repos.
However, if you change to Virtualmin 7 repos for existing installs, for example, by running a new version of Virtualmin 7 install script with flag -s (to setup repositories), then it will eventually break things, as a stack package would be upgraded, forcing MariaDB instead.
That will most definitely break database service and would require a manual fix.