How could iOS (definitely) detect if a battery is chemically degraded? If they slow down performance based on the "expectation" that device is old and it's batt

How could iOS (definitely) detect if a battery is chemically degraded? If they slow down performance based on the "expectation" that device is old and it's battery _must have_ degraded, then it is obviously a slowdown for many unsuspecting users, whose batteries might be in fact absolutely fine. I have SE and never had any shutdowns; fortunately staying on 9.3.3, without any need nor desire to upgrade. I'd be utterly disappointed if next software update started to "manage" performance for me, suspecting old battery.