Cannot enable innodb_read_only on slave

I am trying to make a MariaDB slave instance read only. From what I understand, read_only option is not truly readonly as users with super privilege can still write to the database.

super_read_only is not yet supported by MariaDB, but there is an open feature request for it.

I also came across innodb_read_only. But innodb_read_only is set to OFF, even when I add -innodb-read-only=1 to mysqld arguments.

Is innodb_read_only allowed on a slave server?