How do you permanently delete your purchase history

You can't, you can only hide items. If you truly want to delete all evidence of purchases you would have to ask Apple to completely delete your Apple ID and everything associated with it.

Hide and unhide music, movies, TV shows, audiobooks, and books - https://support.apple.com/HT208167 - "Hiding an item doesn't hide the record of purchase." Hiding them means they will not show in your available downloads list but they will still appear in your purchase history. Has to be done on a computer through iTunes.