I assume he means something like turning off the JS JIT, not turning off JS completely. IIRC iOS turns off Safari's JIT when in lockdown mode. Ladybird browser

I assume he means something like turning off the JS JIT, not turning off JS completely. IIRC iOS turns off Safari's JIT when in lockdown mode. Ladybird browser also abandoned its JIT apparently due to security concerns. JS JIT is one important example but also in general if you write your code to only focus on correctness and not performance then you will get safer code (all else being equal).