Inexplicably, Apple's Reminders app to this date has not bothered to offer us an option to delete all of the completed reminders at once. It should, at the very least, be able to remove completed items after a set number of days, but even that isn't allowed. Basically, you're left with two choices—either, joy, delete completed reminders manually, one by one, like an animal; or, leverage the power of scripting and iOS automation to your advantage like a boss.
Shortcuts Focus: how to delete all your completed reminders in one fell swoop
