I just checked and noticed the change too. And I’m like you, I want to respect the Open Graph image recommended format, and not waste a CMS field + have to create a specific image, just for nodes to have an OG illustration. I regularly default nodes OG image to the homepage, standard, site-wide OG image.
You can still use custom code to add whatever static OG image taken from your Library or outside. Use this code in the Head section.
<meta property="og:image" content="http://domaine.com/ogimage.png" />
You should go to the wishlist and add an idea to bring the support for static image back. I’ll vote for it.
If I republish the sites I’ve been using a static image for CMS nodes (all of my CMS sites I think) then I’m losing the OG image for all my nodes. That’s some change…