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Dual Class cards were from specific sets. Don't think about it too hard, there were neutral cards for things like secret synergy as well. There will possibly be cards from other classes with Holy tags, like Renounce Darkness. That doesn't mean you're forced to use Spirit Healer in Warlock, but there's no reason not to have Spirit Healer in neutral because it's not really a Paladin or Priest thing even if Holy usually is.
Congrats, you successfully identified the only collectible Holy spell outside of Priest and Paladin!