Sorry, but that example you found there is the implementation of a dereferrer and would not hide the referrer when clicking e.g. on that external link you just posted except if you had manually added the https://www.example.com/hide-referer? part in your post.
If the Discourse team comes to the conclusion that they don’t want that feature (with or without the optional part), I will have to accept that. But there’s still a chance that they like it (and implement it) or at least don’t mind, so that it could still be implemented as a community PR or hypothetically as a plugin (which would need groundwork though).