Over at Six Colors, Dan Moren has some valid criticism about the lack of removing completed tasks from Reminders on the Mac and iOS, a feature that is available at iCloud.com:
Last week on Clockwise, we discussed how we use Apple’s Reminders app, and I lamented that there was no option to have old, checked-off reminders automatically discarded. If I look at my Shopping List, for example, there are more than 1100 completed reminders that I really don’t need to keep around. But there’s no easy way to get rid of them, except to go through and delete them one-by-one. Which I’m not going to do.
Listener Stephen pointed out, however, that there is an option to remove all your old completed reminders—it’s just not in an obvious place, because it’s not available in the macOS or iOS Reminders app.
That’s right, it’s on iCloud.
Inconsistency between iCloud and its Mac and iOS apps isn't anything new, but it's surprising that such a feature still doesn't currently exist.
On the Mac front, Dr. Drang has offered up an AppleScript solution to accomplish this. Using Keyboard Maestro, it can also be scheduled to run automatically.
For those of us who mostly use iOS, removing completed reminders is easily done with Workflow. You can create a simple workflow that contains just two actions: Find Reminders Where and Remove Reminders.
The first action should be customized so it is only looking for completed reminders. You can also add another filter to restrict the results by different attributes, or even by list. The second action deletes all the reminders that meet the previous action's criteria. Before doing so, it displays a warning that contains the total number of reminders it's going to remove.
Unlike Dr. Drang's Mac solution, there isn't a way of scheduling scripts or workflows to automatically run in iOS. A workaround is to use the Share Sheet of the workflow to add it to Reminders, editing the details to make it a recurring task. This creates a reminder with a link directly to the workflow that you can tap. Whenever the reminder is due, you can tap the link icon to open Workflow directly on the workflow, ready to be run.