How does it determine what "80%" is ?
Resistors used like that are notorious for being both large, and getting hot when used. Both things that are tricky to cram into a small handheld device.
You also have to wonder what the point of doing all this faffing is. Mobile phone batteries are optimised for energy density above almost anything else, which comes with tradeoffs, include much more rapid natural degradation overtime that can’t be reduced or limited by gentle charging cycles. A phones battery is basically guaranteed to be unhealthy after 4-5 years regardless of what you do or don’t do.
Just because two batteries have the same basic chemistry (Lithium-ion of example), doesn’t mean their behaviour is the same. There’s a lot of possible customisation and tweaking within that single chemistry envelope.
My personal experience is that modern devices are far better about battery management, my phones used to wear out much sooner than 5 years back when smartphones were a new concept and Li-Ion was new and fancy tech. So all that "faffing" seems to be doing something useful, because even my quite old and forgotten tablet that I forget to charge seems to be doing quite well.