Apple doesn't need to break backward compatibility to block Beeper, they just need a way to fingerprint traffic from Beeper which is going to be trivial unless

Apple doesn't need to break backward compatibility to block Beeper, they just need a way to fingerprint traffic from Beeper which is going to be trivial unless Beeper finds a 100% on-device solution. Even then, it'll still be pretty easy because Apple has trillions of interactions with Apple devices to analyze and compare against Beeper.