How to recommend that a question is better suited for another SE site that isn't listed in the close reasons?

Question

Today I found MO questions where commenters recommended that Proof Assistants or History of Science and Mathematics would be preferable to MO. Other better placements I remember include Mathematica, Theoretical Computer Science, and Mathematics Educators.

I can understand not wanting to make a very long list of suggested alternative SE sites, but are the listed options reconsidered on a regular basis and updated accordingly? And what is the best approach to suggesting a site that isn't listed?

Answer 1

From the answer to the FAQ post "What is migration and how does it work?" (on Meta Stack Exchange, here is a link to the current revision):

If the site isn't in the list of available migration targets, or the question got closed without being migrated, first read the below points and consider if migration is/was really the appropriate action (in many cases, it isn't). If you think so, you can flag the question for moderator attention and request that they migrate it for you. Simply explain in the reason which site you want it to be migrated. (Please specify only one site; don't list all possible sites it could go to.)

Moderators have a possibility to migrate to any site. (Regular users can only choose one of the default migration paths.) In both cases, only questions which are at most 60 days old can be migrated.

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Some additional links:

• One can find some useful links also in the tag-info for the tag migration.
• There are a few similar questions on this meta: Recommending to move the question to another site or Redirection for off-topics posts. In both cases, there are no answers, only comments. (There is also the deleted question List of close reasons. It is available to 10k+ users - and some of the comments can be found in the MathOverflow chatroom. Several of the questions linked there discuss adding various migration paths.)
• The choice of the default migration paths was discussed here: What outgoing migration paths should we have? Several questions linked to that one are discussing other possible migration targets. Here is a related post by Jon Ericson on Meta Stack Exchange: When should we consider adding a default migration path?
• Since you have mentioned Proof Assistants, I will point out that beta sites cannot be added to that list.
• There is only a limited number of slots that can be added to the list of default migration paths. (At most four in addition to meta, according to the answer here: When voting to close, only 3 alternative SE listed as options. Again, several of the questions linked there seem to be related. Although it seems that in exceptional cases, a site can have more: Should some communities have more than 4 migration targets?)
• And one can look at various questions tagged migration for other related discussions on this meta.