If a Mastodon server closes down, members can download offline post archives, but there's currently no official way to display these archives online.
A proposed solution by @oliphant is a static website generator they have created called "Posty", which turns Mastodon public post archives into standalone static public websites:
️ https://codeberg.org/oliphant/posty
It could be useful for servers that are closing down, admins could perhaps offer this as a service to their members.