On mobile, searching inside playlists recently turned into an option in the three-dot menu on the display page. I miss the old interface where you just drag down to expose a search field, focus it and start typing. :(
I still have that on iOS? Never noticed it going away and it’s still there for me. That feature really did take way too long for me to realize although.
I really should have said Android. I guess I just assumed they used a cross-platform framework on mobile, with no justification at all. Oops.