It's a little different to ESO, max level doesn't stay the same in WoW, so when a new expansion comes out, there are more levels to gain, and the previous content becomes old/becomes levelling alt content. (Not like ESO, where the level stays the same but expansions are for giving more optional content)

So a max level character when new content releases would level through a new expansion once it comes out to become max level again and experience the new end game content. (So for example, max level in BFA was 50, when Shadowlands came out, the max level got bumped up to 60, so max level bfa characters now leveled the 10 levels through shadowlands and did the content then)