Classic Editor Addon by SO WP & Greg Schoppe - Classic Editor plugin doesn't remove Gutenberg by default. With this addon function we set the option that controls this from no-replace to replace, we remove the Settings link from the main Plugins page and we hide the Settings from the Settings > Writing screen. We also suppress the Nag screen tha…

classic-editor-addon.php

<?php
/**
 * Plugin Name: Classic Editor Addon
 * Plugin Author: Pieter Bos (https://so-wp.com) and Greg Schoppe (https://gschoppe.com)
 * Description: The Classic Editor plugin doesn't remove Gutenberg by default. With this function we set the option that controls this from no-replace to replace and we remove the Settings link from the main Plugins page
 */


function classic_editor_addon_hardcode_replace( $value ) {
	return 'replace';
}

function classic_editor_addon_remove_settings_link() {
	remove_filter( 'plugin_action_links', 'classic_editor_add_settings_link' );
}

function classic_editor_hide_settings() {
	?>
	<style>
		#classic-editor-options{display:none;}
	</style>
	<?php
}

function classic_editor_addon_init() {
	/**
	 * Remove the Try Gutenberg Panel, slated for WordPress 4.9.8 
	 */
	remove_action( 'try_gutenberg_panel', 'wp_try_gutenberg_panel' );
	if ( function_exists( 'classic_editor_init_actions' ) ) {
		/**
		 * Change the default option of "no-replace" to "replace",
		 * which means the checkbox will be unticked and the plugin
		 * does what it says from the get-go (L336). 
		 */
		add_filter( 'pre_option_classic-editor-replace', 'classic_editor_addon_hardcode_replace' );

		/**
		 * Remove link to the settings from the Plugins screen (L277).
		 */
		add_action( 'plugins_loaded', 'classic_editor_addon_remove_settings_link' );		

		/**
		 * Hide the Settings of the plugin on the Settings > Writing screen
		 */
		add_action( 'admin_footer', 'classic_editor_hide_settings' );
	}
}

add_action( 'plugins_loaded', 'classic_editor_addon_init', 1, 0 );

Available as official plugin now: https://github.com/senlin/classic-editor-addon