By all accounts, Apple is developing foldable iPhones and would release one when they are comfortable with it, when they feel it has the least disadvantages compared to a traditional slab phone.
Blackberry, Palm, Microsoft, and Nokia, on the other hand, were blindsided by the iPhone and Android, and pretty much I’d say they were all years behind where they should have been in developing an iPhone competitor when the iPhone came out. In some cases, a real mature iPhone competitor from those guys took 5 years after the iPhone launched. On the other hand, it’s been 4, going on 5 years after the Galaxy Fold (September 2019). I don’t think folding phones has had that iPhone moment yet, they’ve been a tiny amount of sales of Android phones, never mind phones overall. Are they going to grow, yes, but Apple will have an easier time making a foldable iPhone than Nokia in making their iPhone competitor.