I found a copy of the Nissan Leaf 8 year battery warranty online, and there's nothing in it about having to keep the battery between 80 and 20%. Clearly, it's good advice, and if you don't need the range that day, well worth doing to maximise battery life on any Li-ion EV. However, I think it's perhaps a bit like ICE vehicle manufacturers warning you not to run out of fuel. Doesn't do them any good, but they still have to honour the warranty. I've seen something in the Leaf FAQs section, I vaguely remember, about saying not to fast charge to over 80%. Again though, I think the battery management software ought to just stop that happening automatically.