Is it not possible to put clickable links in signatures?

I see a few people (Reinheart comes to mind) every so often with actual clickable links in their signatures, but when I try to use the BBcode [URL] tag, it says it's disabled.

Was it something that was enabled once upon and time and was since removed, and those that had URLs prior to the change get to keep them? I'd like to put the two URLs in my signature as clickable links rather than hiding them in a spoiler.