What do you mean by detaching the fork?

You can send a ticket to GitHub support to have a project appear to have no parent project (detach the fork)

Gotcha. Thanks for the explanation!

I think that's a terrible idea, disregarding the origins and removing an element of credit and attribution to the original authors. Be proud of where you came from, don't try to hide it.

The problem is that the original is defunct but "fork" on GitHub assumes that the fork is going to merge changes upstream.

Credit goes in the credits file, not polluting every interaction with the project.

Plus I believe there is at least one way in which forks may be removed en masse if the original is removed.

What pollution? It's just a link to the project's origin.