When I try to post a link to github, I get this. Since a lot of extension development goes on in GitHub, I think it would be appropriate to allow posting links there.
Oh goodness. Okay, looking to see where that setting lives in Discourse. 
It’d also be great to know which host it’s talking about - I had a couple links and needed to figure out which one was bad.
Are all of the links you’re attempting using the .com TLD, or a different one?
The only one I’ve had issues with is github.com, but only when creating a new “topic”, not when replying. It allowed docs.nova.app
Okay. I think that it should now be working.