WordPress REST Response as a must-use plugin, inspired by https://deliciousbrains.com/comparing-wordpress-rest-api-performance-admin-ajax-php/

mu-example-rest-response.php

<?php

add_action( 'muplugins_loaded',function() : void {

	$my_rest_endpoint = '/wp-json/super-admin-all-sites-menu/v1/sites/';
	$request_uri      = parse_url( $_SERVER['REQUEST_URI'], PHP_URL_PATH );

	/**
	 * Bail if not the correct request.
	 */
	if ( $my_rest_endpoint  !== trailingslashit( $request_uri ) ) {
		return;
	}

	/**
	 * Get HTTP Raw POST data.
	 */
	$params = json_decode(file_get_contents("php://input"), true);

	/**
	 * Bail if not an array, and not the correct request.
	 */
	if ( ! is_array( $params ) && ! isset( $params['offset'] ) ) {
		return;
	}

	/**
	 * Sanitize the request.
	 */
	$offset = filter_var( wp_unslash( $params['offset'] ), FILTER_VALIDATE_INT, [ 'default' => 0 ] );

	/**
	 * Your code here.
	 * 
	 * e.g.:
	 */
	$offset = $offset + 100;

	/**
	 * Set the response.
	 */
	$response['response'] = 'success';
	$response['data']     = $offset;

	/**
	 * Send JSON response.
	 * wp_send_json() sets the correct HTTP headers and stops the execution of the script.
	 */
	wp_send_json( $response );

} );