A foldable iPhone has been pretty firmly rumored for fall 2026 for a while now, I think you can take it as virtually certain: https://www.macrumors.com/guide/foldable-iphone/

I don't think the Air is merely a precursor to the foldable, however. It's rather that the technological evolution that allowed for the 5mm thin iPad Pro last year allows for the Air this year and helps with the foldable next year (and thinner MacBooks are rumored as well). But the Air is probably there to stay as a separate form factor, assuming that it doesn't flop in terms of sales. The foldable will be much heavier and bulkier, more resembling a Pro Max.