I don't think you should delete them. Maybe replace them with more up-to-date, refined information. I know that takes some work and that it's a hassle, though. So maybe it would work to update the recipe pages with a minimal amount of detail and provide links to the best, most detailed threads for each style.
From what I remember, even years ago, I occasionally found my way to some of the pizzamaking.com recipe pages, but I never stuck around because my pizzamaking skills were already beyond what I saw on the recipe pages. Because of that, I assumed the boards were not very useful to me; that they were probably more appropriate for people who were less experienced than myself. So I never looked at the boards. However, I now realize the boards were much more sophisticated and more helpful than what I saw on the front of the site. As a result of joining the forum, my pizzamaking skills and knowledge have increased exponentially.
I'm sure there are plenty of other people out there who, similarly, would feel at home on the boards but have never bothered looking at the boards because the front of the site is so outdated.