For someone looking to learn a graphics api to write a 3D renderer as a beginner (me), who has no experience with rendering or rendering API’s is plain opengl still worth learning or should I be looking into the Google angle library instead?
If you decide learning GL itself, then it does not matter which one you learn, as modern core profile GL and GLES2/3 versions are almost the same. They have very minor differences. GLES2 is almost like GL 2.1 with old FFP functionality removed. And GLES3 is almost like core profile GL 3.3.
The point of why I am suggesting to use Angle instead of regular GL is that it is much more stable and tested across various driver. With regular GL you'll find that you have a ton of bugs and smalls differences on many different PC when you're shipping your application. Which makes writing and debugging GL code very hard.
That said, if you're on Windows, then I would strongly recommend using D3D11 instead. The API is way better, and it has less driver issues.