Get rid of debian-sys-maint instead of root in MySQL 8
Solution 1:
So long as the root MySQL account uses unix_authauthentication plugin (so no password required for root user to log into MySQL) then you should be fine to remove the debian-sys-maint user.
Note that as of Debian 9/Stretch by default, it no longer exists of you install MariaDB from the default repos.
If you don't have unix_authauthentication enabled for the root user, it may still work ok, although some automated functionality may no longer work.