Alacritty uses OpenGL ES 2.0 which is not deprecated. It's shipped OOB in macOS through the ANGLE project by Google, mainly because Safari depends on it to provide WebGL support (it's kind of OpenGL ES for JavaScript). ANGLE translates OpenGL ES so that it runs on top of Metal, D3D, or desktop OpenGL.

OpenGL ES 2.0 is probably the most portable GPU rendering API there is, it pretty much runs anywhere. That said, using Metal or D3D should definitely bring noticeable improvements, especially when it comes to memory bandwidth (eg. on Metal no need to send pixels to the GPU memory for atlas-based text rendering).