I see lots of questions about Drupal, and there is a SE site for Drupal.
Should I flag those questions to be moved there?
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)”.
Beta sites might be deleted in the future, and I think that is the reason why migration there is disabled.
But in my opinion, it should be enabled.