Hmm, my Lumia 1520 uses a Snapdragon 800 SoC, and it is running Insider build 15063.2 perfectly fine.

But isn't this build (15063.2) already the Creators Update? And if so,
why remove support for phones that can clearly (and already do) run the Creators Update just fine?

I could understand removing support on devices where the Creators Update doesn't run well,
but removing it from devices where it's already running fine, is a bit of a weird decision from Microsoft.