They probably should not remove them and instead mark them as outdated
F-Droid
F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
Matrix space | forum | IRC
F-Droid has a setting that allows you to hide apps with anti-features. One of them is 'source code no longer available' but another for 'source code not updated in 2+years' would be great for this
Why not remove them? They take server space to host, and show up in searches when they arent useful.
Archiving and data hoarding purposes
They still could be moved to the archive repo
One reason might be that someone looking to create an app might want to use it as a starting point... It would make them easier to find if they don't remove them from f-droid
You can submit take-down requests by starting a new issue on our issue tracker. (This requires a gitlab.com account.)
That only appears to be if the app violates copyright or by the author. Not because an app is outdated.