I do not know about the throttling on and off button, but I don’t believe that app will give such an option to iPhone users. When your battery is degraded it can’t perform that well. So whenever certain tasks or apps on your phone need a lot of processing power your battery isn’t able to give the power the processor asks for, so your phone turns off for protection. What happens is that people think that their phone is broken and send it back to apple. Apple then needs to look at it and sends it back and says “get yourself a new battery”. This whole process is costly for them, so they throttle your processor down so that it won’t turn off out of sudden and you don’t bother them with these problems.