When WordPress 4.9.8 hits, it will include a callout to try Gutenberg. It’s a great way to bring it to the attention of those who may not already be aware of it. However, for those who already are, it may be unwelcome.
It can be dismissed easily but if you’re managing a number of sites you may not want it to appear in the first place, save getting a lot of questions from users.
There is a plugin available to ensure it doesn’t appear but, tbh, it’s a lot simpler than even that.
Just add the following line to your theme’s
|remove_action( 'try_gutenberg_panel', 'wp_try_gutenberg_panel' );|
And, well, that’s it.