How certain are you that X-Forwarded-For is ignored by default? This guide isn’t particularly new, this one only sets X-Forwarded-For in the HTTP-only case, and I haven’t seen any complaints about rate limits hitting. Also, I’m operating a Discourse site behind such a Nginx proxy, and Discourse does see the real IP address.
If you’re certain, feel free to add the line above – it’s a wiki!