Is there a workaround? This just seems like the worst reason not to use an operating system…
Your ISP (and/or VPN provider) has been banned from nixos.wiki due to ongoing attacks originating from their IP space.
To regain access to the wiki, you can:
- Use another internet service provider.
- Contact your ISP and urge them to take action that will result in a stop of the attacks.
Maybe just use https://wiki.nixos.org as it is the official wiki while https://nixos.wiki is run by a different part of the community. Because the official wiki is basically a fork of the inoffical one the content should be the same (plus some additions from the wiki team).
Yeah, nixos.wiki is long abandoned, though it’s not as if the official wiki is in much better shape, you may as well just ask your questions on this forum.
The workaround I use is to access it via Tor.
nix-shell -p tor-browser
Super! thank you all! 
Nixos.wiki is “blocking entire Autonomous System Numbers (ASNs)”, as mentioned on their “News” section on their frontpage. Mullvad exit node are probably within those AS, unfortunately…
"Additionally, we’ve heightened Cloudflare challenge levels. We apologize for any inconvenience this may cause our users. Until we compile a comprehensive list of the attacking networks, these measures will remain necessary. "