User blog comment:BlackWizardZeref/Fairy Tail Chapter 394: Juvia vs Keith Review/@comment-25111584-20140725120319/@comment-25244540-20140725123317

I think Lost Magic is labeled as such because the knowledge of it isn't available to the public as a whole. If there's a way to teach someone it or find and learn how to use it then its not as strange.

For example, some of the Dark Guild had all its members use the same form of magic meaning it was known to the public. Dragon Slayer magic is not public due to a dragon having to teach you how to use it.