I think they will never implement it. If all the shit and stuff you've already watched was filtered out and every movie was only listed ones you'd likely see a fairly empty page.
I think there's a large enough repository that there would always be something to show. Why wouldn't Netflix prefer to show me things I might want to watch (even if it's a low probability) over things I said I definitely don't want to watch.
For me, I think it basically just reduces my engagement. If my recommendations are filled with things I've already decided I don't want to see, I'll just go do something else.
