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

/**
 * Classic Editor Addon by SO WP
 * 
 * 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
 * 
 * Add this to your theme's functions.php file or functionality plugin
 * Released by Pieter Bos of SO WP (https://so-wp.com) 
 */
if ( function_exists( 'classic_editor_init_actions' ) ) {

	// Classic Editor is active, continue

	/**
	 * Change the default option of "no-replace" to "replace",
	 * which means the checkbox will be unticked and the bloody plugin
	 * does what it says from the get-go (L336). 
	 */
	$option_name = 'classic-editor-replace' ;
	$new_value = 'replace' ;

	if ( get_option( $option_name ) == 'no-replace' ) {
		// The option already exists, so we just update it.
		update_option( $option_name, $new_value );
	}

	/**
	 * Remove Settings link to the settings from the Plugins screen (L277).
	 */
	add_filter( 'plugin_action_links', 'classic_editor_remove_settings_link', 10, 2 );

	function classic_editor_remove_settings_link( $links, $file ) {
		if ( $file === 'classic-editor/classic-editor.php' && current_user_can( 'manage_options' ) ) {
			array_shift( $links );
		}
		return $links;
	}
}

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