they can right now, we have an Ads anti-feature, they’ll get NonFreeNet for the connection, nothing stops the dev
See how Cryptomator, FairEmail and Netguard do it. there are more examples. (read Paid features in opensource apps )
Making it clear in app and on website how developers can be sustained, links to their donation platforms and etc. Now we’re working on integrating Taler
just for F-Droid, see text above