Update: The criteria we've been following in practice, might not be precisely what we've described "on paper" here. Hopefully one of us will have time to sort this out on Meta sooner or later! We've never seen a complaint about anything we've been doing so far, as far as I know.
Result: Questions satisfying the below criteria (and even if they don't satisfy all of them), are closed by moderators when you click "flag" under the question, select "in need of moderator intervention", then type "abandoned". Just one flag is enough if the question is clearly "abandoned", but a comment is usually left informing the user what happened, and that if they "come back" then the question can be reopened. The questions get closed as "needs detail for clarity" since the user has not responded to comments. I think it was in a chat room, that Tyberius said he didn't want to use one of our 3 possible "custom close reasons" on this, so the questions get closed (by use of a flagon the question) as "needs detail for clarity" instead of "abandoned question". Users interested in changing (or removing) this policy should probably start a new Meta discussion. Questions can also be closed if the user has not been online on the SE network for 6 months (or whatever "last seen 6 months ago" means), since even if the question were to be answered they are unlikely to see it, though they're always welcome to come back and let us know (i.e. get the question reopened).
There's users that create an account, ask a question, but never come back. Such users often have not responded to any of the questions, suggestions, or requests for an input file or more details, and maybe never will (since they aren't signing in to see them, and don't have any other network account from which they might get pinged to see comments).
I propose to add a close vote reason, under "community specific guidelines", for abandoned questions, that following the following criteria (the exact details can be changed as we gain experience):
- The question has no answers (mandatory for this close vote reason; since if there's answers it might not need closing, and alternative close reasons can always be given if necessary).
- User hasn't been online since the week that they joined the site, and >2 weeks have passed.
- There's comments that ask questions or give suggestions, which are not responded to by OP.
- It is unlikely for the question to be answered in the absence of OP coming back and responding to the comments.
Such questions can always be re-opened when the user comes back and answers the comments, or if someone miraculously comes along saying in the comments that they want to answer the question. Remember we are only using this close vote reason in the case where "it is unlikely for the question to be answered without" some more information, so this is highly likely, but if that does happen, we can re-open the question.
This is one of the rare times I'll say that it's okay for a moderator to "hammer" the question closed, because if the user is no longer visiting the site, then it doesn't matter if the question is open or closed or has answers or doesn't have answers (except if the user is visiting the site to check for answers, without signing in; but one criteria is that there has to be comments that are not responded to, and if they're checking for answers but not responding to the questions, then it's bad practice anyway). Moderators should then hammer the question back open if the user comes back.
This is not just about keeping the percent answered high: Abandoned questions can actually be harmful to many people, since people might waste their time trying to solve the problem and write comments about how their own solution did not work, not noticing that the user hasn't been active since the day the question got asked 2+ weeks ago. Today someone wrote a rather thoughtful and detailed comment with a suggestion to try a different Gaussian keyword, and might not have noticed that the comment is unlikely to be seen any time soon since the OP has not been active since the week they made their account.
A relevant quote from here:
"Abandoned, unanswered questions can be a nuisance for readers when they appear in search results. While every question deserves a chance to be answered, at some point the annoyance to those searching for a solution outweighs the increasingly-small chance that an answer will be provided."