As RelatedSites extension has been undeployed from Wikimedia projects
today, a question arises: who will maintain it in the future? Unless
someone volunteers to maintain it, we will probably eventually have to
archive the repository. According to WikiApiary, besides WMF it's used
only on vojnaenciklopedija.com (ping Zoranzoki21).
Lots of extensions are maintainerless. I dont see why that implies it will
eventually have to be archived. Perhaps the day will come when it no longer
works with modern mediawiki, but until that day comes, why not leave it be?