This is not an SEO topic, per se, but a forum policy discussion relating to ANOTHER FORUM, not this one. A URL is always a URL whether a link is attached or not. Just hilite any text URL in Chrome and right click: Go to example.com. If their browser can distinguish a URL in a body of text, then so can their robots.
Members who join the forum (any forum) for the sole purpose of getting the URL out into the ethersphere generally stand out, after a time, or sometimes right away. Forums have policies against this sort of ABUSE because their members object to it. Here at WPW we respond to members in ways we hope will be helpful, and will benefit the community as a whole.
When we say we join to help out, and then accumulate a mass of posts proving it, we are being good to our word. When we don't amass any posts in 7 years of membership, it says another thing altogether. We don't see any senior members griping about signature policies on this or any other forum. It's only ever an issue to the low performers--aka, the less helpful among us.