On Thu, Jun 17, 2021, at 16:23, Stephan Hoyer wrote: 
This happens all the time. Even if we make copy='never' an error *today*, users will be encountering existing versions of NumPy for years into the future, so we won't be able to change the behavior of copy='never' for a very long time. Our deprecation policy says we would need to wait at least one year for this, but frankly I'm not sure that's enough for the possibility of silent bugs. 3-4 years might be more realistic.

If we go the enum route, we may just as well deprecate string arguments at the same time so that we have the flexibility to introduce them again in the future.