There are many closed source applications that have been removed from the Google Play catalog for various reasons.
Often this reason is the cessation of further development and maintenance of the application.
Why don’t these people make the code open to everyone if they don’t need it anymore?
This could bring great benefits to all people, but everything that was once created simply disappears.
F-Droid isn’t the only one: I also like Neo Store, which has more than just the F-Droid repository and I think has a better interface. However, I think not all repo are 100% safe because F-Droid verify all the apps they have while Neo-Store doesn’t because it simply list known repos, which you can activate however you like in the settings.