Is there a flag or config similar to allowUnfree or permittedInsecurePackages that allows this?
I’m not certain about this, but I think in this case the package is just straight up removed from Nixpkgs. If you can provide the name and error message it should be easy to check.
In this case if you still want it, it would mostly boil down to the same as installing a package from an old Nixpkgs, for which you can find various guides depending on your use case.