Here's an article on the subject:
This article states that there have been no formal controlled studies on the impact of pumpkin/squash seeds on worms in chickens. However, there is an amino acid in squash seeds, Curcurbitin, that has been shown to kill worms in test tubes. There has also been some research on pumpkin seeds as a dewormer in horses and humans. It's not just a myth, but there is no definitive research that shows that it works in chickens. However, there is apparently a lot of anecdotal evidence that it does.
Thank you! Good to know for the future.