This question already has an answer here:
When a beta site is closed, the questions that can be migrated are given to other sites. But what about the questions that aren't relevant on other sites?
Let's say you have a question, that was fairly high voted, and has an accepted answer. That question may be useful to someone else in the future, even if it will not be useful to as many people as a StackExchange site requires. If that question is deleted, than in the future, people who are searching for answers on a question that happened to be covered by a closed beta, will not be able to get it.
What I propose, is that when a beta site dies, instead of deleting it, it should be turned into an "archive site".
The archive site would basically be the same as the regular site, except there would be a big "Archive" at the top of the site (it would look just like the meta site, except with "archive" instead of "meta", and it would be colored even more gray), and it would be read-only. People would be able to view the questions and answers, but no one would be able add anything, or vote.
Also, I would suggest that StackExchange keep track of the traffic that comes into that site. If for some reason, the topic of that dead-beta become of interest to enough people, the site could be reopened.
Any questions, comments, suggestions?