The indestructibility mechanic took some serious templating.
At first, indestructible was a keyword mechanic, meaning that it would be like Severed Legion or Darksteel Colossus or Birds of Paradise. So a card like Darksteel Pendant would have been templated initially this way (shown here with a first pass at some reminder text):
Indestructible (This card can't be destroyed but can be sacrificed or removed from the game.)
, : Look at the top card of your library. You may put that card on the bottom of your library.
Indestructible (If this card would be put into the graveyard from play, instead leave this card in play. This ability does not work if this card is sacrificed.)
(Note #1: This playtest card, a 2/2 indestructible artifact creature called "Indestructo," never became a card in a real Magic set. Note #2: indestructibility was once going to be included in "Bacon," a.k.a. Mirrodin, but was pushed off into Darksteel.)
As Randy Buehler's January 9 article mentions, using "indestructible" as a keyword caused some weird phrasing. Cards would "gain indestructible until end of turn" (via Slobad, Goblin Tinkerer's ability, for example), which just didn't sound natural.
So "indestructible" is now templated the way "Neurok Spy" is, as a regular English adjective that has specific rules implications.