I've seen it in a couple software developer contracts here in Norway. I find it despicable and have always gotten it removed from any contract before signing. I don't get why it's even legal to have in contracts. I certainly hope it's unenforceable.

It depends on the juridiction, it might not be legal or enforceable in Norway but it definitely is at least in California.

There is the famous lawsuit of Mattel suing Bratz, on the basis that the Bratz creator started to work on his new dolls while being employed by Mattel.

I'm not sure how it ended, but it wasn't dismissed right away and they spent years in court.

It’s very enforceable.