A form of distraction-free i.e. full-screen post editor existed in some form from many years in WordPress, that has now been forced to default.
If you don’t want to use it on your website, you can simply disable it with the use of a plugin, just as we did while switching back to Classic Editor which is fully supported and will be maintained until at least 2022, or as long as it is necessary.
Disable Fullscreen Editor Mode is a lightweight plugin that you simply need to activate in your WordPress site and that’s it. The plugin has no settings, it automatically works for posts, pages, and other custom post types.