@Silverfish If the question is otherwise following the guidelines for routine bookwork style questions (at least those under homework in the [help/on-topic], the tag isn't so crucial (useful, but not critical); on the other hand if the question doesn't follow the guidelines it should close with the specific close reason for those posts whether or not it has the tag.
So if the guidelines aren't being followed the OP needs to edit anyway, and they might as well add the tag themselves when they do. (Adding it to a closed question doesn't seem to have much value until it's being fixed anyway). If the guidelines are being followed, the tag could be added.
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