The SD Association has this page explaining the speeds, and from what I can see, it’s just to guarantee speeds with certain bus types. Most devices currently use a UHS-I bus, and the older speed classes cap out at pretty pathetic numbers, so they’re kind of useless. For the steam deck, any of the higher-end cards are going to be about the same, and the IOPS are going to be more important in terms of loading times anyway.
Everything except for the UHS-III part, yes. Some UHS-II stuff exists, but I think it’s limited to full-size SD cards right now, and the Deck only has a UHS-I bus.