Not in the sense that the card got worse. When you owned for instance Force of Nature, you owned a 5 mana spell that summons three Treants, that you could use in Wild, Duels, and some Tavern Brawls. And because you owned that card, you were given for free a copy of the Classic version of Force of Nature, that cost 6 mana, and gave the Treants charge and a severly limited lifespan.
The Force of Nature that you owned is still there, and it is still usable in Wild, Duels, and some Tavern Brawls. And likely also in some future seasons of Twist. It is unchanged, and not nerfed.
The free extra “Classic” copy that you had may or may not still be in your collection, I don’t know. However, since it was specifically for a game mode that has been retired, that is no longer relevant. You won’t be able to play with it, because it was allowed in Classic only.
So no, the cards were not nerfed. A game mode was retired, that made a bunch of cards unplayable, but those cards were all given for free, as “copies” of cards you had in your collection. Basically, they were loaner cards.
That said, this technical explanation does not do justice to the fact that there are people who bought packs and/or crafted cards specifically because they wanted to use the Classic counterpart of that card in their Classic deck. If you crafted Force of Nature for the express purpose of using it in Classic, and no intention to ever use it in any other game mode, then you have been cheated.
Classic was never announced as a temporary mode, and players could reasonably expect that their investment in crafting cards for Classic decks would be useful forever. That expectation has been thwarted, by the unexpected retirement of Classic, and I do agree that it would be fair to think about some sort of compensation.
But what? That is the problem. You may have crafted Force of Nature for a Classic deck. I perhaps crafted the same card for a Wild deck. Should you receive dust and I not receive any? How would Blizzard decide what you and I crafted the card (or bought the packs) for?