When on the Plan to Watch page, goes through and deletes every single entry until there's none left. Should work with the majority of list styles, but could break if the default modern style isn't used. Make sure to disable this script after you're done using it.
< Feedback on MyAnimeList: Mass-delete your Plan to Watch list
Used this script to purge the entirety of my anime and manga lists after altering the code a bit and it worked beautifully. Thank you!