The flag dialog (and I assume close menu) as a whole is unintuitive. We should consider doing massive changes.
Off-topic, to me, means that the question will never be a good question for the site, unless it is completely changed. "Unsalvageable", one might say. (I'm not a fan of the triage button label.)
Then there are questions that are technically salvageable, but only by the author.
Cannot be reproduced fits when you cannot reproduce the error with the 0 lines of code given in the question.
Let me go over the issues with each option under "Off-topic" (for SO). Most of these suffer from being way too broad umbrellas.
You may not be able to get help on [other SE site]
Questions about general computing hardware and software are off-topic for Stack Overflow unless they directly involve tools used primarily for programming. You may be able to get help on Super User.
Did you know that Super User considers Facebook questions off-topic? I certainly didn't because I haven't read their help page. But I felt so certain it would've been on-topic because of all the "general computing" questions that I've flagged.
This text is just causing the crap to get up and circulate through the SE system. There is also a bit of overlap between sites, as being on topic one place doesn't mean that you're off topic everywhere else.
I tend to use this reason for two things: hardware problems and "normal people" problems.
Asking for recommendations
Questions asking us to recommend or find a book, tool, software library, tutorial or other off-site resource are off-topic for Stack Overflow as they tend to attract opinionated answers and spam. Instead, describe the problem and what has been done so far to solve it.
I think most of these questions fall under the umbrella of "opinion based", which isn't even in the off-topic menu (for some reason). One can argue that all spam (by the SE meaning) centers around pushing the opinion "this is great, you need it" onto others. The opinion-based reason should be expanded to include recommendation questions.
Why isn't this working?
Questions seeking debugging help ("why isn't this code working?") must include the desired behavior, a specific problem or error and the shortest code necessary to reproduce it in the question itself. Questions without a clear problem statement are not useful to other readers. See: How to create a Minimal, Complete, and Verifiable example.
If adding more code invalidates the close reason, the question isn't actually off-topic.
It's unlikely to be helpful
This question was caused by a problem that can no longer be reproduced or a simple typographical error. While similar questions may be on-topic here, this one was resolved in a manner unlikely to help future readers. This can often be avoided by identifying and closely inspecting the shortest program necessary to reproduce the problem before posting.
This is actually more of a reason to delete something after a certain period of time. The wording would indicate that you shouldn't use it until a solution is found, which is somewhat counterproductive.
The ability to migrate should be migrated.
First of all, the "too old to migrate" appears next to "off topic because...", which lead me to believe (at first) that I was suggesting migration every time I selected the "you may be able to get help on SU" option. (I thought that the "belongs on another SE site" was an overflow menu. I didn't realize at first it vanished when a question's too old.)
I think that migration should only be possible after a post is closed. Again, there's no reason it can't be on topic on two sites. It might be a good idea to show excerpts from the help page of each site, too.