I suspect that one reason why forks used to have drain holes was to make it easy to tune the damping by changing the viscosity of the oil, to adapt to different road conditions or different tracks. Nowadays, most high-performance and racing bikes have adjustable damping, so there is much less need to tinker with the viscosity. Perhaps the drain holes were never intended just for changing out the oil because it's "dirty".
Speaking of "dirty" fork oil, I agree that with a completely sealed fork system (especially on bikes with fork bellows), there's little possibility that the oil can become contaminated. The "dirt" in the oil could be graphite or molybdenum disulfide particles that are often infused into the seals and are sloughed off over time. As we know, these materials are very slippery, and probably won't affect the function of the oil or the damping mechanism.
