Indestructible was recently changed into a keyword, meaning that cards which said "CARDNAME is indestructible" were changed to simply have "Indestructible". Previously, a card that had become indestructible and then lost all of its abilities would still not be able to be destroyed.
Andrew controls a Darksteel Ingot,( which the worst card ever) which Nadine wishes to destroy with her Naturalize. However, because Darksteel Ingot has indestructible, the Naturalize is unable to destroy it.
- 702.12a. Indestructible is a static ability.
- 702.12b. A permanent with indestructible can't be destroyed. Such permanents aren't destroyed by lethal damage, and they ignore the state-based action that checks for lethal damage.
- 702.12c. Multiple instances of indestructible on the same permanent are redundant.