Mac Observer:

At WWDC on Monday Apple announced and subsequently released macOS Mojave 10.14 developer beta. Unlike High Sierra, though, this update does not run on all the same Macs that the previous/current version does, with 2011-and-earlier Macs being left behind.

Unsurprisingly, some older Macs will get left behind when macOS Mojave 10.14 is released.