GL_DEPTH_CLAMP has no effect (reverse z)
It is compatible, but instead of clipping it now clamps (hence the name). If the depth of your primitives is outside the [0,1] range it ends up behaving as if you have no depth-test at all. The only way to mitigate that is to either scale down the depth range so it fits in [0,1] or to use GL_NV_depth_buffer_float to disable clipping and clamping completely.