Seems overly complex to me. If you’re handing out API keys, you have the developers e-mail addresses.
Log requests sent to deprecated end points, and have your system drop said developers an e-mail once a week or so letting them know they need to change their API integration before such and such date. Then obviously, update docs and throw in some warning message within JSON response of said end points.
Easy and straight forward without adding in new HTTP headers then hoping developers integrate with them, etc.
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