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Tests.

Okay, so first you have to have a whitelist of "allowed" usages of depreciated functionality for this to actually work. Anything that makes it into prod goes on the whitelist. All dev environments, QA, and your automated testing run with fail mode on and trying to push code that adds to the whitelist should set off alarm bells. From there it's up to the team / business if it's worth doing it anyway.

You use it when testing and you can turn deprecation flags on and off. Even on a single production machine/vm.