Even before the XP buff you could not really finish entire expansions anymore before out-leveling them. The XP buff just makes it "worse" (or better, depending on your point of view).
Shadowlands revamps the entire leveling process, letting you pick a single expansion to level in from start to finish, which allows experiencing the full expansion story in its entirety. The only "downside" being that you only level in one expansion, instead of all of them. But if you want to experience all of them, you can make multiple characters.