A Google+ Page can help you with SEO, but the impact is going to be minimal, if any at all. The Google My Business page is a must for local search. However, they are two different things, as much as Google still tries to pretend they're not.
Gone are the days of Google+ being a must have. Back when it was supposed to be Google's little "Facebook killer," everyone had to have a page. Now that Google has unbundled all of its apps from G+, and bled out the creative team behind the product, this platform just continues to die a very slow death. Common are corporate pages that haven't seen any new content posted in two years. Some use G+ as a content repository, but between YouTube, Instagram, Facebook, Twitter, LinkedIn, and website, what's the point?
There are cases where G+ is useful. If you have an active community on your page, you would want to keep that page, at least until you can drive them to a different platform. It also helps by getting another instance of brand mention into your SERPs, though not many people click on those results anymore, and of those who do, few convert to anything.
Your Google My Business page, on the other hand, is an active business listing. People look at what appears in that listing, mostly in the Knowledge Graph, and will actively seek it out for your business hours, logo, photos, website link, and phone number.
If Google does shut down your G+ page, it won't impact your GMB page, which is a connected but separate entity. However, if you really want to keep your G+ page, you can post a few things, as Michael mentioned above. Since it looks like you haven't posted in a long time - long enough for Google to have taken note - I'd let it go. There's nothing sadder than an abandoned social media account.