Judging by the way it’s designed, I thought it already worked like that (toggle in the middle - filter not applied, to one side to hide, to the other - ‘only show’)
Especially useful for filtering other people’s lists. Personally useful for me in filtering the lists I’ve made for different streaming services I use (which i use as lower priority watchlists, to keep the main watchlist cleaner). In the old website, I could scroll through the list and see which ones have the list button highlighted, hover over it and in seconds see if it’s in my watchlist, in the new version, it takes so much longer.