Recommended chargin limit

I received a notice from Apple recently which recommended max. 80% charging
level for my iPhone for longer battery life. I was just wondering why did
they not program the iOS to enforce this limit? It might also involve some
minor chip redesign, but it shouldn’t be a major update and would let us
leave the phone in the charger without worrying that we will find it at
100%  when we get back to it.