Cool, excellent, great information, but let's pause to note that we're posting on the Touring subforum, and for a cycletourist on a long journey, this crosses the river from 'common' to 'available'. I can get 17" (yes, seventeen inch) tires for my Moulton bike delivered to my door in a day or two as well, but no one would call them common.
I've opined before that if I were bumping down the road on a duct-taped tire knowing I'd have to spend a couple of days in a grim little hamlet in flyover country if I couldn't find a tire in the general mercantile, I'd want to be on 20"ers (ISO406mm). I'd think this would be the case in any country where children rode bicycles or where delivery/cargo bikes were in use.
Yes. I think this has been the point of this thread since it was started a decade ago — helping hardcore tourers decide which size wheel to have on their bikes should they go on a worldwide expedition. Such tour includes developing countries and remote places. For that purpose, 26” (ISO 559) wheel is still king. I foresee for that to remain that way for at least another decade. As mentioned before, 700c tires and replacement parts (i.e., for wider rims suitable for rougher terrain) are catching up quickly though on worldwide availability.