Just wanted to describe my experience with upgrading to iOS13.1:
After I upgraded to iOS13.1 and upgraded Good Task to the latest, I was hit with really sluggish performance IN GT: e.g. tasks taking a minute or more to become visible). I've been using the Reminders on my phone for years, so I definitely have a ton of old completed tasks!
I tried to use the advanced command to remove completed tasks, recommended above, but it didn't work for me. It would either instantly return to the preferences screen without doing anything, or crash the app. I force quit the app, restarted the phone, and tried again with the same results.
What finally worked was replacing the old overweight-with-completed-tasks reminder lists with new ones. I created a new list, and moved my active reminders to it. Then in GoodTask I unselected the old list and selected the new one. Instantly, the app started to behave as expected.