As mentioned in the other comments it's not consistent nor always supported. And you can do the multithreading only by sharing already rendered fbo's.

In D3D it works as OpenGL display lists would, except with arbitrary commands. So you have multiple threads composing the scene and then a single thread just issues few commands to replay the command buffers created by the other threads.

It would be rather simple to implement the same in OpenGL as we already have the display list concept, even if it originally was made to reduce the amount of glvertex3f calls.

"complete lack" was maybe bit of an exaggeration, however in practice it is true.