Comment by jcrites

According to the California Business and Professions Code Section 16600, “every contract by which anyone is restrained from engaging in a lawful profession, trade, or business of any kind is to that extent void.”

https://leginfo.legislature.ca.gov/faces/codes_displaySectio....

This does not make contracts containing such clauses "illegal", but rather unenforceable in court. The student could sign a contract saying "I will not work on a company that competes with Repl.it", and he did, and the company tried to sue him over it with that as the only claim, then the court would quickly grant a motion to dismiss the lawsuit.