Unless you have a specific requirement for it, I wouldn't choose 26" now, though TBH in my large sized frame I've never been keen on the aesthetics.
The size isn't going to disappear, there's so many about that availability will be there as long as there's the demand, 650B never went away, 27" stuff was still findable, just not as easily or with the same choice. I think that's what's happening with 26", volume producers won't put the effort into development and only produce those items worth doing in their production quantities. So for example, the likes of Schwalbe haven't offered a 26" option in their latest tyres, and have probably decided the lower volume of sales warrants a narrower range.
My folder has 24" (507) wheels, I have no doubt I'll always be able to source rims and tyres, I think I have a choice of four tyres and every rim I've seen looks like the same one rebranded! I doubt 26" will ever fall that low, it started from a much higher base.
I have noticed a couple of suppliers have just introduced ac 26" tyre Rene Herse for example .
Yes, all sorts of less common components, inc tyres, will always be available from specialist providers, but you can't pop into Halfords for them and the low production always carries a price premium.