I mean... Ultimately just want to clarify "27.5" bikes are actually abouts 27 inches and therefore incredibly close to 26 inch. If you were to like create a custom bike that retains the exact same geometry spec but fitted around 26 inch wheels, it would just be that same 27.5 bike but with zippier wheels that corner well but tons of feedback and some scary hang ups on the front end (if you were accustomed to a 29er).

And chucking 26s on a 27.5 enduro will have rather obvious results. Pedal strikes, awkward chain stay length, and the same front end hang ups with slightly smaller wheel. Chuck on wider tires and the straight line roll over improves marginally, and the cornering VANISHES.

So, yeah. 26 is close enough to 27 there's no REAL reason to go out of your way to do it specifically for enduro/DH riding.

For FREE RIDING and DJing, of course, it's very valuable and it's actually a struggle many free riders have had as companies discontinue 26 inch downhill frames. See, Remy Morton: https://www.pinkbike.com/news/bike-check-remy-mortons-chainless-mini-mullet-park-bike.html

Biggest takeaway from this post should be, you don't want plus size tires on anything except a fat bike or bike packing type hardtail. The benefit will never be worth the severe weight and cornering cost.

Yes I have thought a lot about this because I love BMX bikes, DJers, old free ride/DH bikes, etc. The reality is 27 is very close. Maybe you will get lucky and find a 27.5 enduro frame with oddly high bottom bracket but short chainstay (unlikely). Then you can chuck 26s on it with less problem. Sounds fun.