The best replacement for caste system is class system.

The best alternatives to caste system are rated as following:
★★★★★  class system
★  racism
★  slavery
★  apartheid
  feudalism
  varna
  class

Your alternative for caste system is not listed? Suggest!
Speaking about caste system, one probably shouldn't forget about hinduism.

Other search terms related to caste system and their ratings are:
★★★★★ hinduism
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caste system
class system
racism