Or even better, (if open-source) put it on F-droid.

Looks like it's preparing: https://github.com/celzero/rethink-app/issues/210

It is open source.

I was thinking of Android software in more general terms.

I think this is because they're probably vulnerable to being kicked off the Google Play store since it conflicts directly with Google's revenue model.

I agree with you though, Google and Apple shouldn't be playing gatekeeper between me and my phone which I bought with my money.

or they should provide a flag to show these apps.