I have read that Core Graphics is based on OpenGL ES and uses the Quartz Drawing Engine on Apple devices (iOS, OSX)
However with the upcoming deprecation of OpenGL ES for Metal, will Core Graphics be updated to support Metal and/or software rendering for coming iOS/OSX devices?
First, Core Graphics doesn't "use" Quartz. "Core Graphics" and "Quartz" are just two names for the same thing. They are equivalent.
Second, Apple doesn't promise what technology Core Graphics uses under the hood. They've occasionally touted the acceleration they were able to accomplish by using some particular technology, but that's marketing — marketing to developers, but marketing nonetheless — not a design contract. They reserve the right and ability to change how Core Graphics is implemented, and have done so often. Any developer who writes code which depends on the specific implementation is risking having their code break with future updates to the OS. Developers should only rely on the design contract in the documentation and headers.
It is very likely that Core Graphics is already using Metal under the hood. It makes no difference to you as a developer or user whether it is or isn't.
Finally, Core Graphics has not been deprecated. That means that there's no reason to expect it to go away, break, or lose functionality any time soon.