That’s a good question, actually. I’ve played a fair bit of FTL and I do think it hews very close to the spirit of a true roguelike, since starting a game with the same ship is always the same experience. I do think meta unlocks change the way you play the game a bit, though, since you may target unlocks and achievements over victory. I’d ultimately put it on the “light” side, but I agree that the game skirts the line.
I haven’t played slay the spire, but I’m thinking of games like Binding of Isaac with a lot of unlocks, and I’d say that those change the game experience quite a bit depending on what you have unlocked.