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.
I see @creativetruth thank you for that too.
That made me worry because our email accounts are very important for our other important accounts in many sites that we use.