As an example, Infinity for Reddit is likely no longer viable due to the Reddit API changes. This version of Talkback has not been updated since 2013 while this version says it is deprecated. The only viable one at the moment is this one since it has the most recent update.
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