how do i delete app download history

Delete from where/how ? You can't delete any item from an account's purchase history, if you don't want apps to show in the Purchased redownloading area all you can do is hide them : Hide apps for your iPhone, iPad, iPod touch, or Mac - Apple Support

It's never been possible to permanently delete purchases, possibly due to not wanting people to do it accidentally.

What happens when you try to hide them via your computer's iTunes ? There was a bug yesterday that was stopping it working, but that has apparently now been fixed.

It doesn't need to be linked to a computer, the hiding process doesn't need it.

I can hide apps from the Purchased tab in the App Store app on my iPhone 7 (iOS 11.2.6), swiping from right-to-left gives me a 'hide' button which removes it from that list. That is what you are doing on the phone ?