Here's a discussion from 2015 about Robert Plant's current signing habits, with a few autographs on page 2. The two people who got them signed said if he signed, he only personalized them. Of course, there are exceptions to every rule, but I've heard that before.
Here's another discussion with a genuine Plant in one of the replies signed in about 2011, posted by Terrier. The Plant in the original post is not one most of us considered authentic.