I have hundreds of playlists so every time I want to add a song it takes me years to find the right playlist on Android.
That's the reason I always used the desktop version, when you right-click > "Add to playlist", if you started typing, it filtered the list of playlists shown!
The latest desktop version has removed this functionality as a regression. Please bring it back! It is essential to me. And adding it to mobile would make the mobile app usable to us, power playlists users.