We use it as a joke name for functions/modules that are deprecated or dangerous, or that otherwise should be avoided. It works pretty well even though it's (probably) an empty threat.
Also, I take it that the described duplication is due to isolation and compartmentalization (which I can appreciate). I'm curious if some kind of "final dedupe" could be tacked onto the end of the build process that just runs a bunch of fixed algorithms on the assets to find and remove duplicate resources - and maybe even code fragments, like Courgette (https://www.chromium.org/developers/design-documents/softwar...) does.
FWIW, doing something like this would probably take a lot of pressure off the dev teams and make everyone's lives easier. On the current track, the app will be 300MB in a few months. lol
The same reason why you otherwise deprecate stuff. Because it's still used. As for React, they have such a variable to allow interop with the react dev tools.
In a twisted kind of way, I'd call that a feature.
If you're aware you don't know what you're doing, but still want to do it, that's kind of ok. Problem is when you're not even aware you're about to jump into some pretty hot water.