I am sure they always will make tires in that size, but some of the ones I might want to buy might only be available in other sizes. My folding bike takes a very odd size,I have bought several spares to keep on hand.

But 26 inch was common enough, that some will always be available. I am not stocking up on lots of spares, I have several more years worth of that size already in storage. I have three bikes in that size.

A neighbor is a bike mechanic at a large shop near a large university campus. A lot of the bikes you see on campus are decades old, my neighbor was telling me that they keep a good stock of 27 X 1 1/4 inch tires for that reason, but you won't find any other store in town that has any in stock, nobody has built a bike with that size in quite a few decades.

If you bought a touring bike that is 26 inch, you might want to carry a spare on any tours you do. A lot of shops won't have a spare of that size.