Here's a general overview of the maximum level caps introduced by each expansion up to that point:
Classic WoW: Level 60
The Burning Crusade: Level 70
Wrath of the Lich King: Level 80
Cataclysm: Level 85
Mists of Pandaria: Level 90
Warlords of Draenor: Level 100
Legion: Level 110
Battle for Azeroth: Level 120
Shadowlands: Level 60