Well more of a clarification of how they’re supposed to work.

A recent article from the Official Strategy Blog Site has clarified the following:

While Skill Cards treated as a Spell or Trap Card can be recovered to your hand by the likes of “Mask of Darkness”, you cannot do the following:

You can not have a Skill Card be in your Deck at any time.

This means you cannot shuffle it into the Deck using something like Jar of Avarice.