I see lots of questions about Drupal, and there is a SE site for Drupal.
Should I flag those questions to be moved there?
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If it's a new question, and it's off-topic on Stack Overflow, and you're pretty sure it's on-topic on the target site, then vote to close and flag to request a migration.
If it's an old question, don't bother unless you're a regular on the target site and you're thinking “we (target) want it from them (SO)”, not “they (target) want it from us (SO)”.