In 2026: Please don't learn this; it's obsolete. Learn Vulkan (Or another modern API; or how to build engines on top of them; or how to write shaders; or how to do GPU compute using CUDA etc)
OpenGL is not obsolete and is totally fine for many use cases. Vulkan is way too complicated and cumbersome for many. Even if OpenGL doesn't get new features what exists now will continue to work for decades.
Another (related) problem is that the many sediment layers that have accumulated over nearly three decades are not clearly separated (and Vulkan is starting to suffer from the same problem btw, there's always at least five different ways to do the same thing, three of which are outdated or not recommended to be used on specific GPU architectures, and the other two have complicated relationships and interdependencies with other redundant or optional features, it's apparently some sort of Khronos curse to always create the biggest possible mess when it comes to 3D APIs).
On Windows (maybe even on Linux via Proton), starting with D3D11 makes a lot more sense, or on macOS with Metal v1. Both APIs are 'close enough' to modern 3D APIs to carry a lot of the knowledge over, but without being too low-level like Vulkan or D3D12, WebGPU running in browsers is also a good choice (even though it inherits some bad design decisions from Vulkan 1.0 when it comes to resource bindings, and is in some places actually slower than WebGL2).