This decision was made for a number of reasons, but in large part due to the fact OpenGL is inferior to the MetalAPI in regards to performance and efficiency. You can still work with OpenGL in 4.14.3 and prior engine versions, but moving forward we did not find it necessary to support that API any longer.
“Metal, Apple’s groundbreaking graphics technology, accelerates Core Animation and Core Graphics to boost system-level rendering by up to 50 percent,* and efficiency by up to 40 percent,** resulting in faster graphics performance for everyday apps. Metal also takes full advantage of your CPU and GPU, delivering up to 10 times faster draw call performance for a richer, more fluid experience in games and pro apps.*”
Thank you,