Patches for Android 4.4.4 and 6.0.1 are not provided by ASB (Android Security Bulletin) anymore.
This means: security wise Android 4.4.4 and 6.0.1 are formally dead.
Nevertheless there are a few enthusiasts, still backporting security patches from recent Android versions.
You should know, such backports are no formal ASB security patches.
Therefore the security patch level of this ROM is named as "Unofficial Security Backport".