bookreader Why do so many manufacturers have options to limit charge to 80% though? MacBooks and iPhones both limit charge to 80% for as long as possible (until they think you're likely to remove them from the charger). OnePlus phones, Pixel phones running stock firmware, and Samsung phones all have this by default as well.
Why does everyone do it if it's "not a thing"? Everything I've read says that batteries typically degrade more quickly as the voltage is higher, and many manufacturers seem to believe this to be true as well. While I do trust the charging circuit to behave sanely, the charging circuit ultimately has to pick some voltage and say "this is the top." I might want the top to be somewhere else. As you said, the chemistry doesn't understand percentages, so there's no magic, 100% agreed upon value for max charge voltage. This is why it makes sense for it to be configurable. If the chemistry knew about percentages, it wouldn't make any sense for it to be configurable, because there would be a universally known "correct" top voltage. But there's not.