I just gave the search a try as well and the mix up is really annoying indeed. 😜
I'm pretty sure that an PCI 1.0 or PCI 2.1 card will work in a PCI 2.2 slot, but I'm not sure whether it will work as well the other way around. 😕

Does PCI 1.0 really exist in the wild? The oldest I've seen is 2.0. If 1.0 exists, then it must be relegated to some obscure intel or OEM board. I seem to recall 1.0 was defective in some regard, perhaps lack of busmastering?
From what recall, the PCI slot is designed so that the older cards will normally fit into newer slots and vice versa, but if there is any electrical incompatibility the card should be keyed in such a way by the designer to prevent it from being inserted into the wrong type of slot slot. As to whether or not some low voltage PCI cards were designed with incorrect keying and could potentially be fried by going into a 5V slot, I have no idea, but it is certainly a possibility.

I always thought that old pci 486 motherboards would have 1.0 since they were the first ones to get pci slots, but do they actually have 2.0?

1.0 doesn't exist in commercially available stuff, 2.0 is the fist thing available. 2.2 cards do not seem to work in earlier things without explicit support even if voltage support exists.