The parameter innodb_idle_flush_pct that was introduced in MariaDB Server 10.1.2 by MDEV-6932 has no effect ever since the InnoDB changes from MySQL 5.7.9 were applied.

I suggest that we mark this parameter deprecated in 10.2, 10.3, 10.4 and declare it as removed it in 10.5. The page flushing code was rewritten in MySQL 5.7 and later in MDEV-23399 and MDEV-23855. It could be not worth the risk and effort to try to resurrect the parameter in these GA release series.