The question A confusing use of a negative seems like it belongs on English Language Learners. I was considering flagging it for moderator attention and choosing a custom reason to say this. But I'm unsure if this is the proper course of action because it's probably not off-topic on ELU (or is it?) and because ELL is a beta site. So should I flag cases like this or leave them be?
It is OK to flag a question and suggest a migration to another site. If we mods think it should stay here, we'll let you know. Otherwise, we'll check with the mods on the other site and see if they support the migration. If they do, we'll send it over to them.
Since the user who asked that question said to be learning English, the question could be a candidate for migration. That doesn't mean it will be migrated; poor questions are generally not migrated. (The motto is, "don't migrate crap.")
Flagging for moderation attention to suggest a question to be migrated is fine. The moderators normally consult the moderators of the other site, and if they don't want the question to be migrated because there is already a similar question, or because it is not a good fit for the site, then the question will not be migrated.
The purpose of flagging a question for migration is to put it on the BEST site, beta or not. If you honestly think that a question should be on site X and not ELU, then flag it. And let the moderators of the two sites decide.