I ran a large online community with tens of thousands of users that existed for nearly 19 years. When it came time to close it down, I left a notice on the site for several months before i made it read only...
then the helpful guys at archive team[1] helped me create a complete archive of the site on archive.org with their irc bot.
once that was complete, i replaced the site with a notice and linked to the archives from there.
As an aside, one of the old domains that we used for a while lapsed at some point, and some spammer put up a copy of the pages from the internet archives with ads injected into the content. That eventually went to an SEO landing page a few months later.