Common practice in many MMOs.
Rift. Lot of specs are behind a paywall and even roles for specific classes. Eg Mage Tank is paid.
Guild Wars 2. Revenant is behind HoT as are Elite Specialisations.
WoW. Expansions based classes.
FF14. Expansion based jobs.
And so on. The Warden and Necro do come with Morrowind and Elsweyr respectively but they are also made a separate purchase for people who may just want the classes.