When wanting to do a selective restore of a backup taken with mariabackup DISCARD TABLESPACE / IMPORT TABLESPACE is used after preparing the backup with mariabackup --prepare --export

When having a SEQUENCE backed by an InnoDB table the related .ibd tablespace file can't be discarded though:

MariaDB [test]> alter table test_sequence discard tablespace;

ERROR 1031 (HY000): Storage engine SEQUENCE of the table `test`.`test_sequence` doesn't have this option

So there does not seem to be a way to restore such a sequence from a physical backup like mariabackup.

IMHO the SEQUENCE engine here should not just outright refuse such ALTER attempts, but pass them through to the underlying engine actually used for sequence storage.