At the top of the page https://meta.wikimedia.org/wiki/Special:SiteMatrix
is shown the user interface message https://meta.wikimedia.org/wiki/MediaWiki:Sitematrix-summary

Two suggested newline additions to that message:

Links in red mean the project does not exist in that language yet. See https://meta.wikimedia.org/wiki/Requests_for_new_languages for information about that process.
Links with strikeout mean the project did exist but has been closed. See https://meta.wikimedia.org/wiki/Proposals_for_closing_projects for information about that process.

(partially from a suggestion by @Thadguidry on wikidata-l@)