It would be very easy to implement in software. Just display a green and yellow/red bar on the battery status icon and allow for people to say full is 80%, empty is 20%. Or something.

For me it is well worth going from charger to charger (they are everywhere, especially if you put some wireless chargers here and there) if that means the phone can still do a full day after 5 years.

I generally use phones until they stop working. I'm into my 3rd year with my iPhone 12 mini, the phone is still super fast, takes great pictures and still feels like "my new phone".