Do all Jehovah's Witnesses participate in shunning former JWs or is that something that is optional?

I don't think so. I think the Watch Tower probably tells them to, but I doubt they can enforce it. Some parents apparently choose to do what the WT tells them to do. I asked former JWs what their experience was like and one said his father has all but left. He said he still takes his father to the Kingdom Hall because he likes to be around the people even though he no longer believes what they teach. That's an indication that they're not always as strict about shunning.

On the other hand, people have reported losing everyone they'd ever known and loved because they isolated themselves from the outside world and then were shunned. It apparently can be a very traumatic experience for some people but not so bad for others. It might depend on how involved one's family is in the group.