The "secret" behind Secret Paladin (other than all the secrets) was their perfect curve:
Secretkeeper - Secret Pally announcer. The only relevant 1-drop that they can play. If they have a few secrets in their hand, they can buff it up to a 2/3 or a 3/4 on turn 2, but most likely they would just play...
Turns 1-5 were designed to be as difficult to deal with as possible, and as soon as MC dropped, the Paladin basically becomes invincible for the rest of the game.
So, as you can see, pretty much all of the turn 1-5 minions rotated out of standard. Pally's early game is weak now, and has a much harder time contesting the board without Minibot/Shredder/Belcher. So now Paladin rarely goes into Turn 6 with board control, which really hurts the Challenger turn. Avenge certainly was a powerful secret, but even if it was still in standard, Secret Pally would have a hard time keeping up.