A number of projects were closed/deleted but are still dumped regularly, e.g. https://rm.wiktionary.org (https://dumps.wikimedia.org/rmwiktionary)
Those projects should probably not be dumped anymore.
The difficulty is to have an up to date list of such projects. A good start would be this category: https://meta.wikimedia.org/wiki/Category:Approved_proposals_for_closing_wikis or this page: https://meta.wikimedia.org/wiki/Proposals_for_closing_projects.