I just learned (from Maciek at the OpenStore telegram group) that the app still needs to be modified, OpenStore won't accept a click file for 16.04 because it targets a different system version, even if you can install it manually.
He pointed me to the UT App Dev telegram group and advised me to make my own modified version.
Unfortunately, I really don't have the time.
If anyone who reads this has time to make and publish a version for 20.04 on OpenStore?