On a related note, it should be possible for Mariabackup 10.3 to create a backup from a running MariaDB Server 10.2.
Currently, both these are refused due to redo log format tag mismatch. We should remove the refusal and test the upgrade.
The format tag will still serve its main purpose: preventing an older version of MariaDB from starting up from newer InnoDB files. Even that one we could theoretically relax if desired, in a separate issue: allow 10.2 to start up on files left behind by a clean shutdown of 10.3.