Thanks for bringing this to our attention.
If you would like the Search function to be reimplemented within playlists, make sure to head on over here and add your vote to the idea. The more votes that the idea garners, the more likely it is that Spotify will consider reimplementing it!
In the meantime, you can check out this post (which includes screenshots) to see how to access the Search function for playlists.
Hope that helps! Let us know if you have any other questions regarding this....