Well, what was the first game with unlockable content? What even IS unlockable content?
If you play Super Mario Bros., you “unlock” world 1-2 by beating world 1-1.
RC Pro Am you had to earn money to buy/unlock better parts and cars.
SSF2T had a cheat code that let you play as Akuma.
Wolfenstein 3D and DOOM Shareware demos you only got one mission and had to pay money for the rest of the missions.
In SNES JRPGs like FF6 and Chrono Trigger you don’t necessarily have to get all the characters in your party and such. You also don’t have every character from the start. You “unlock” them as the game goes along.
In Final Fantasy 1 you unlock the silver sword by grinding a whole bunch.
On a pinball machine you literally unlock multiball by achieving certain goals.
Just about everything in video games can be considered unlocking.
The only problem is when it’s involved in asymmetric multiplayer. All players should have equal access to all races, classes, units, weapons, fighters, cards, etc. Extremely not cool if different players have different options based on how much money or time they have invested. See: CCGs, Smash Bros, MMORPGs, etc.