[kevincox]

I suspect even if you weren't using heuristics you would be best to pick a different static filter than "None". For example I would expect that "Up" does much better on average as you are essentially bringing in context that the compressor would struggle to line up. I think most encoders would probably only use "None" in cases where heuristics show that nothing else helps.

So it is probably mandatory because only a tiny minority of images wouldn't use a filter. So it is better to just require it to avoid one more condition in the decoder.