There are many F-Droid repositories aside from main F-Droid. Please see here: https://forum.f-droid.org/t/known-repositories/721

https://guardianproject.info/ one is particularly interesting. Haven is a Guardian Project app.

Haven might not be in the main F-droid repo(?), but you can add theirs to install it. It’s also on the google play store and the source is available to build it yourself.

https://guardianproject.github.io/haven/

So it's actually part of the guardian project, which is a separate repository of also open-source apps and you can enable the guardian repository inside of f-droid and then it will be installable.

Based on the wiki it's because the last build of it failed for whatever reason.