559 (26inch) diameter wheels are not dead.

[Mike Ayling]

Just saw on CycleBlaze that a bloke had purchased a new Nomad and it still comes with 26 inch wheels!

All hail to Robin Thorn!

[Matt2matt2002]

Just back from Thailand and plenty there.
Not sure of the global situation, though.

[mickeg]

A neighbor is a bike mechanic at a large bike shop near a large university campus.  They sell lots of 27 inch tires.  I can't remember how many decades it has been since bikes with those tires sizes were sold, but it has been a long time.

I am sure you will always be able to buy 26 inch tires, but the selection will get worse and worse over time.  Quality rims will also start to disappear.

[Andyb1]

SJS list 69 different types of rigid 26 inch tyre and 17 different types of folding 26 inch tyre.
(When counting  ‘types’ I am including different sizes of the same model of tyre).

Edit to add:
Unfortunately new designs of tyres like the Schwarbe Efficiency seem not to be made in 26 inch.