I’m vegan for three years and vegetarian for around six years. Some time ago I bought a pack of fake chicken gyros and to me it tasted so close to real chicken that I somehow felt disgusted by it and couldn’t eat it. Before, I always liked chicken and also other animal products. But somehow I got not only used to resist the temptation but somehow conditioned myself into completely changing my preferences. I still buy fake grated cheese quite often but in terms of meat alternatives I prefer patties, sausages, falafel and stuff that taste nothing like the original.
So if I compare my personal ranking to yours, my top tier is probably “A” and your “S” in many cases would be just as bad as “F”.
From my perspective mails are federated. If I want to explain federation as a concept to someone I always use mail as an example because everyone can write to everyone independent of the provider, you can selfhost it easily, you could move from one company to another (if you use your own domain), protocols are all FOSS.
So at least it’s an open and distributed system. What would be missing for it to count as federated?