Just announced MacOS Mojave: what level of support for Metal is required?

The article Apple Outlines Metal-Capable Cards goes into the detail needed.

From linked article --

According to Apple, the following graphics cards are known to be Metal-capable and compatible with macOS Mojave on the mid-2010 and mid-2012 Mac Pro models:

• MSI Gaming Radeon RX 560 128-bit 4GB GDDR5
  • SAPPHIRE Radeon PULSE RX 580 8GB GDDR5
  • SAPPHIRE Radeon HD 7950 Mac Edition
  • NVIDIA Quadro K5000 for Mac
  • NVIDIA GeForce GTX 680 Mac Edition

Metal is required to run the OS. Apple has deprecated OpenGL and OpenCL and additionally is now using Metal exclusively to render the GUI.

If your Mac does not support Metal, the user interface will not be able to run, and as a result Mojave will not boot.