MariaDB [test]> set global enforce_storage_engine=MyISAM;

ERROR 1228 (HY000): Variable 'enforce_storage_engine' is a SESSION variable and can't be used with SET GLOBAL

I truly hope it's an oversight, although from the last comment to MDEV-6858, it seems it might have been done on purpose. It's understandable why a super-user should be able to overrule it sometimes on the session level, but the main goal of it is still being a server-wide rule.