MoltenGL proprietary and not the only implementation of the GL library on top of modern APIs. There's also ANGLE.
It worked pretty OK-ish in my project running OpenGL game on Windows Phone 8. I was using Microsoft's fork [2].
Another alternative is GLOVE [3], which is a software OpenGL on top of Vulkan.
Another interesting project is WebGPU [4] which targets to provide Vulkan-like APIs across various platforms in browsers.
As it's offshoot which is not quite related is GFX-rs, which became a cross-platform Rust library providing low level graphics API (HAL layer).
looking forward seeing ANGLE getting Metal backend, which been just a promise so far.
[1] https://github.com/google/angle
[2] https://github.com/Microsoft/angle
[3] https://think-silicon.com/products/software/glove/
[4] https://en.wikipedia.org/wiki/WebGPU
[5] https://github.com/gfx-rs/gfx