Either it is or it isn't. If it is, a lot of search engines are in big trouble. Google links to almost every piece of illegal content on the web.
You also have to think carefully about what a link is. Is a magnet link - essentially a hash of the content itself - a link? If so is posting the checksum of a piece of software copyright violation?
Unfortunately the law isn't so black and white, and depends a lot upon intent. Thinking about the technicalities isn't so important; what matters to the court is whether the individuals involved intended to commit a crime.
Upthread people are mentioning images of child sexual abuse.
It's important to mention that in the UK this is an absolute offence. If the image is in your possession then it doesn't matter how or why it's there - intent is irrelevant.
This is important for people responsible for various servers, for example.
Intent still matters. For instance, say someone emails you a image and then contacts the authorities. If merely having such an image in your possesion was an automatic "go to jail, do not pass go", you can be certain there would be a lot of politicians and other unpopular individuals being led away in cuffs.
You either possess the image or you do not. It is a strict liability offence.
There are narrow exemptions - "I did not view the images and I did not know they existed" is one; "The images were unsolicited and I did not keep them" is another.
