While the main CPU (APU) enforces the rule, the Syscon chip (a smaller microcontroller) holds the truth about your firmware version. Advanced modders have learned to:
To downgrade a PS4 from firmware to 9.00 is technically possible but practically prohibitive for 99% of users. It requires micro-soldering, hardware programmers, advanced hex editing, and accepts a 40-60% chance of turning your console into a paperweight. ps4 downgrade 13.02 to 9.00