Jeremy Fink
Jeremy Fink thanked Joy Twain's answer

From what I've heard, they're just considered too old for investing in further updates -- well, at least not major ones.  They will still get security updates.  I'll try to find a link for you.

(Edit)  Here's one:

https://www.slashgear.com/no-android-oreo-for-the-nvidia-shield-tablets-14503959/