For some unknown reason, Apple has never added a native way to remove completed reminders from their Reminders App. Over the past few years, I have completed over 2,500 reminders and never deleted them. To delete them manually would have taken forever. Fortunately, with the new Shortcups, I found a way to automatically delete all completed reminders in the mater of seconds.
How to Delete Completed Items from Reminders App:
First, make sure you have the iOS Shortcut app installed. Then, go to the shortcut app on your iPhone and search for “Clean Completed Reminders”. Once you run the shortcut, it will prompt you several times to make sure you want to delete all completed reminders. Just to be clear, there is no way to recover them once they are deleted so be careful.