The JNIDetachReleasesMonitors flag was added to JDK 6 when it was realized that the VM was not compliant with the JNI specification regarding the release of monitors when a JNI attached thread detached - see JDK-6336479. The flag allowed for restoration of the incorrect behaviour if an application was somehow relying on it.

By now all applications should themselves be JNI-spec compliant and this workaround should not be needed, so we can obsolete the JNIDetachReleasesMonitors flag in JDK 9 (accept it but take no action) and expire (remove) it in JDK 10.