I think people are confused because of the advice to not charge above 80% on fast DC chargers, but that is about the exponential reduction in safe charging spee

I think people are confused because of the advice to not charge above 80% on fast DC chargers, but that is about the exponential reduction in safe charging speed at high state of charge, not battery degradation (excepting old versions of the Nissan Leaf with bad battery management). If you fast charge to above 80% you are wasting your own time and the time of other people waiting to charge, so manufacturers usually quote fast charge times for a 10% to 80% charge, which is both more realistic and make the cars look better.