iOS does have a feature that throttles the CPU on deeply worn-down batteries to save battery life, however, after it got some publicity a few years ago they made it user-toggleable from the settings menu (Battery > Battery Health & Charging)

My XS is currently at 74% of initial capacity (it tells you this too, which is really nice), and I don't even have the option to enable throttling yet, so it must not kick in until things get really dire