Big changes in the WordPress core around mid-December as they updated from 4.9.8 to 5.0 (and shortly after to 5.0.1 and 5.0.2, and now 5.0.3), which includes the Gutenberg editor.
As of this writing, we have not moved some of you to the 5.x series. Most of you hosted at Sustainable Sources have been upgraded and we added and activated a plugin which will allow you to either use Gutenberg or the page editor you’re familiar with. To switch it off and try Gutenberg just go to Plugins => Classic Editor and click the Deactivate link. To go back to Classic, simply click Activate. Classic Editor will be supported for ~2 years, but you may as well learn Gutenberg now.
To get you started, here are some helpful Gutenberg resources:
Gutenberg Handbook – official WordPress guide with an in-depth look at how the editor works and tutorials on block creation. https://wordpress.org/gutenberg/handbook/
WordPress Gutenberg Guide – an illustrated guide by codeinwp that explains how Gutenberg works and shows you some advanced tricks. https://www.codeinwp.com/blog/wordpress-gutenberg-guide/