Disable WordPress REST API completely

disable-wp-json.php

<?php
/**
 * Plugin Name: Disable REST API
 * Version: 0
 */

// completely disable wp-json access
add_filter( 'rest_authentication_errors', function( $access ){
    return new WP_Error( 'rest_cannot_access', 'Bye', array( 'status' => 403 ) );
} );

// also remove actions added by wp-includes/default-filters.php
remove_action( 'wp_head', 'rest_output_link_wp_head' );
remove_action( 'wp_head', 'wp_oembed_add_discovery_links' );
remove_action( 'template_redirect', 'rest_output_link_header' );