iPhone 8, iOS 17 & AppleWatch?

Confused that pretty much everyone is saying that iPhone 8 is "too old" for iOS 17. But, if that is true [and people seem to be very sure about it], does anyone know why I was able to update my iPhone 6s+ to iOS 17??

And, btw, same watch I have used for years was working with iPhone 6s+ years before I finally updated that phone to iOS 17, after which it continued to work.

But then I decided to start using new iPhone 8 that has been sitting around for years. Being older than iPhone 6s+, certainly didn't expect any issues. But then discovered I can't upgrade past iOS 16! And, now, all of a sudden, I can no longer pair my watch with my phone!

Makes no sense, especially since Watch worked with much older hardware and software. Assume there's some logical explanation, but, if so, certainly nothing obvious!

Any ideas?