There is a built-in charge limiting feature in Android now, so that doesn't need any apps, but the only options are limit to 80% or 100%. But for something like this, how else are you going to get your phone to tell a charger when to turn off? Apparently, making an API that allows devs to reliably stop charging in software is too risky, so IoT devices with companion apps are the solution, of course. Not to mention the device manufacturers are probably quite happy to have batteries degrade significantly after warranty ends.