Apple has finally admitted they throttle older iPhones with iOS updates when they release a new model. As the battery degrades over time it’s no longer capable of providing as much power as the CPU needs to draw at peak performance and this can cause unexpected shutdowns so they’ve just been quietly slowing down their phones without disclosing it until now. Probably to encourage you to just buy a new phone for $1000 instead of having the battery replaced for $80.

Anyways if you’re able to replace the battery yourself iFixit sells a kit for $24.99