Thanks for sharing your concerns regarding Sitemap.xml missing out Shop pages, leading to SEO issues. Glad to share that we have implemented sitemap.xml to include Shop pages, starting a few weeks back. To make shop pages part of sitemap.xml, you would need to republish your site, since the introduction of this capability. After that, any new product added to the store would make it to the sitemap automatically.
Let us know if you have any issues after you republish the site.
Product Manager, GoCentral Online stores