In reply to Tammie Lister.

Thanks Tammie for clarification and I'm glad that users won't need to install a plugin to use their metaboxes. This part of the post was a bit confusing:

"For developers who are worried about the compatibility of their metaboxes, Mullenweg said a plugin will be available for providing the legacy edit page for metaboxes."

I assumed that this means metaboxes won't be available in Gutenberg unless you install the plugin and it seems others understood the same. But I'm really glad that this won't be the case. :-)