Agree. But compared to the new APIs, OpenGL is better in that regard. I would argue that legacy OpenGL is even better.

You can implement in software whatever you want and just handwave specific parts to GL, like rasterization. The rest of the boilerplate code will be minimal.

In new APIs you are bound to write 100s of lines of boilerplate code to get to the point. In the time you should be learning about barycentric coordinates, you are still learning about descriptor sets. And still, you don't learn about rasterization, texture mapping, clipping, face culling, because they are still fixed pipeline.

The only problem with learning with 100% cpu rendering is that you need some patience before seeing beautiful stuff because you need to learn/implement everything up until that point.