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It doesn't appear this tag is even being used for the intended "code review" purposes anyways. I can't wrap my head around why this question is tagged with it...
I don't think we can completely eliminate the tag. Like latex and similar tags that now have their own site, there are older questions in code-review that were once on-topic on Stack Overflow, but will never be migrated to the new site.
The idea of a 'deprecation' feature came up as a consequence to the homework tag cleanup - it would block a tag from being used but not require its removal on existing questions prior to saving them after an edit. I don't think it's been officially proposed, but this now provides two good examples of where it would be useful. I might write a feature request for it later if nobody beats me to it.
code-review
is automatically migrated to the Code Review site?