indicate and point to the correct apk-repository at the website
i just found an issue which is or will be somehow related but nothing similar: fdroid-website#57 (closed)
there are some apps which are only available at the archive-repo like https://f-droid.org/packages/com.ghostsq.commander.sftp/, https://f-droid.org/packages/net.osmand.parkingPlugin/ or https://f-droid.org/packages/net.osmand.srtmPlugin.paid/ for now.
thankfully they can be searched for and are shown at the homepage, but there are 2 things which should be addressed IMHO:
an indicator in which repository it (or to be precise every apk or at least the latest/starred apk) can be found would make sense. maybe a hint per apk like in the fdroid-app, but if so with an additional easily visible "general-indicator" for the the latest/starred apk to notice that i have to activate an other/additional repository after installing fdroid.
since i have to deal with and assist not so tech-savvy users which were really desperate and nearly leaving fdroid, because they couldn't find a desired app to install on a new phone but it was clearly visible on the homepage, this will be a huge relief.
and therewith we are at the second point which is more of an existing bug:
at the website of the corresponding "archive-app" all apk-links for sideloading point to
https://f-droid.org/ARCHIVE/<app-name_versioncode>.apkand therefore you only get an 404-error. - so also sideloading also didn't work for the pitiful noobs.
i'm pretty sure a bunch of gitlab-issues are only created because of these issues, like fdroiddata#823 (closed) or fdroiddata#822 (closed), fdroiddata#909 (closed), fdroiddata#908 (closed) or fdroiddata#888 (closed).