Not a stupid question at all, and the answer is that yes, you CAN use them!
Cactus has not banned or retired cards from being playable in tournaments, you can play any card ever printed (though some may have updated abilities, which are posted on these boards if that is the case).
What you saw is that the sets those are from are out of stock. While there are some packs around for Womens and Warriors, for instance, Cactus no longer has a supply and does not sell them.
While a lot of the older cards may not be as useful in today's meta-game, there are some of those older cards that still see constant use (and can be more valuable due to rarity and the fact that their packs are no longer available in large numbers).