I want to point out that although a lot of the discussion is focused on Classic being more difficult to level in than 2019 WoW, to me the big distinction is that combat is much more tactical in Classic and leveling in turn is more strategic. Difficulty is part of the difference, but in my view current WoW has moved closer to being an action game with much of combat being about timing and rotations while Classic WoW still has a significant element of making actual tactical decisions during combat rather than just punching the buttons in the right sequence/right timing.
That to me is the key difference that makes playing Classic better and more interesting. It’s not just difficulty; it’s also how the difficulty difference is achieved.