Hi RDM,
As you mention on this link (Percona XtraBackup Overview - MariaDB Knowledge Base) , xtrabackup is not supported on MariaDB 10.3 and up. By “not supported” it means that the backup will likely fail and you should use mariabackup instead.
For previous versions there are a few gotchas to consider, but was still supported.
In latter versions (let’s say 10.3 and newer), MariaDB has been diverging from upstream MySQL considerably, so not only it’s no longer a drop in replacement (moving from MySQL to MariaDB might be easy, but migrating back might not).
So as a conclusion, because of the incompatible changes and difference on internals, xtrabackup is not longer compatible with MariaDB changes in newer versions.
Regards