The device could track the battery's capacity pretty reliably by measuring inputs during charging and outputs during use. The necessary data is even available to third party apps on both iOS and MacOS (Battery Life, and Battery Health are two examples)

It sounds like the idea is that the OS is throttling the performance to extend the battery life of a battery in poor condition, more than it sounds like it is limiting current draw to avoid "brownouts".

I can see why this is a good idea and a bad idea, in that many people simply want a full day of being reachable on the phone more than they want smartphone features, but others who are considering replacement would like to know that a $100 fix would make their phone less frustrating.

Here's some info from a jailbroken ipad (redacted some unique identifiers):

iPad:~/ root# ioreg -l -w0 | grep Capacity | | | | | "Capacity" = 960 | | "BootCapacityEstimate" = 87 | | "BatteryData" = {"CycleCount"=232,"MaxCapacity"=10639,"FullAvailableCapacity"=11085,"StateOfCharge"=98, "MfgData"=<redacted cuz it looks like a unique id>, "DesignCapacity"=11560,"QmaxCell0"=11678,"Flags"=1,"ManufactureDate"="6205","ChemID"=4384,"BatterySerialNumber"="redacted","Voltage"=4155} | | "CurrentCapacity" = 10500 | | "BootBBCapacity" = 10364 | | "AppleRawCurrentCapacity" = 10242 | | "AbsoluteCapacity" = 11272 | | "AppleRawMaxCapacity" = 10489 | | "MaxCapacity" = 10500 | | "DesignCapacity" = 11560

I hypothesize that the MaxCapacity will drift lower over time. They have all the info they need in their registry.

It is pretty simple to figure out the capacity by monitoring cell voltage and discharge current over time. Monitoring the impedance would be an additional bit of data that would correlate to the health of the materials in the cell, but is neither the easiest, nor the most accurate way of monitoring cell health.