I got that email too. Google+ is their version of facebook. Nobody really used google+ for social media, so they are shutting it down because it is a waste.
All the other google accounts are still widely used, especially their email system gmail. I use it too. I would not worry. The worst thing they will probably do is give you ads or fees in the future, but I doubt that. They don't delete your email.
Youtube is also owned by google. Youtube isn't shutting down either. Just google+. I think they are shutting it down also because people are often confused, and think google is the same as google+. Google+ is just a small part of google that was an unsuccessful business venture.
That made me worry because our email accounts are very important for our other important accounts in many sites that we use.I see @creativetruth thank you for that too.