Thanks for the pointer, @rolko. The contour plugin for OsmAnd~ is indeed one of many apps that were moved to the archive because of weak signature, as discussed in:
Initially it was suggested to submit an issue or merge request to disable a build and later reenable it so that it would be properly signed. However it seems there this catch: the old build will be automatically deleted and if the new build fails you end up having no APK at all for that version of the app (not even in the archive):
App authors were advised to update their app or just bump the version number if they want to reinclude it in F-Droid:
The OsmAnd~ contour plugin is actively developed but only the version code changed over time, not the version name (1.0). It seems odd to bother the app author with the F-Droid signature issue. Extra complication: the F-Droid maintainer for this and many other apps said goodbye to F-Droid last month.