Under what conditions will a proposal become closed? There was not information in the FAQ that helped me discover this, and I thought that it was kind of important to know.
A proposal will be closed if it's still in Definition or Commitment after a year: Closing a Proposal After 1+ Year in Definition or Commitment
A proposal will be closed if it's determined to be a duplicate of an existing site or proposal. For example: http://area51.stackexchange.com/proposals/87060/software-architecture
It is also possible for two proposals to be merged.
There's a new discussion on Meta StackExchange regarding the new rules for beta sites here: Graduation, site closure, and a clearer outlook on the health of SE sites
Essentially, they're no longer closing sites just for being Beta sites any more. They're only closing them now if there are quality problems with the site.