To be clear, I'm just attempting to get a complete idea of the potential lifecycle of a proposal, which includes it being closed. I'm looking for a complete list of vote-to-close and auto-close reasons on Area 51. I've searched around, and been unable to find a this information. clearly if I was able to vote-to-close I'd be able to click the vote-to-close button, and see a list of reasons; clearly I don't have the rep to be able to do that, just pointing out that's one of the places I attempted to find the info; though I did find the info stating 2000+ rep is needed to vote-to-close.
This is the standard set of reasons a proposal may be closed:
Beyond that, we have the following processes in place to work with proposals that are just not going to make it as a Stack Exchange site.
The Final Proposal Review
Closing a Proposal After 1+ Year in Definition or Commitment (link)
In the latter case, it is perfectly acceptable — and sometimes even recommended — to start these proposals again. I would also note that closing and restarting 1+ year old proposals has spurred some of our strongest launches on the network to date!