I think forum URLs are allowed.
I agree about the URLs. Of course you can just hover over a link to see the URL and should do that anyway but often the URL contains information that can help you evaluate if you're even interested in going that far. I've seen a rant on here about people hiding the URL and another about people not hiding the URL so I guess there are differences in opinion. I usually will hide it when the link is only providing additional information not directly essential to what I'm saying. For example:
I filed an issue about this a while ago but never received a response.
vs.
Here's the comment one of the Arduino developers left on the issue I filed: Forum Profile settings disappear · Issue #194 · arduino/forum-assets · GitHub.