This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review-may-2019/3032/159

I'm not sure whether I should change the name to castor-browser, since there's already a different castor packaged by some distros: https://repology.org/project/castor

If you click through to some of the packages in that table, you can see that it's actually picked up a mixture of castor the browser and castor the java library. I think the policy in nixpkgs is probably first come first serve, so as the first castor package, this would get to be called simply "castor". There's no guarantee that the other castor will ever be in nixpkgs either.

I built this and ran the binary. I tested it by visiting some gemini and gopher sites and clicking around.

It's not the most polished of clients at this stage (for me it shows most text as black on a dark background and it panics if you click an https link or an image link). However, I don't think that's got anything to do with how its packaged, so assuming it is useful to somebody and continues to improve, looks ok to me.