Very simple single class for creating custom endpoint in vanilla wordpress. Based on this stackoverflow answer: https://stackoverflow.com/questions/38901536/wordpress-map-custom-url-to-a-function Not intended to be a full solution.

example.php

<?php
/**
 * Set Up Endpoint
 */
WP_Endpoint::add('some-json', 'getSomeJSON');
function getSomeJSON()
{
	header('Content-Type: application/json');

	// some logic

	echo json_encode([
		'some' => 'data',
		'more' => 'data'
	]);
	die;
}

WP_Endpoint.php

<?php
/**
 * @author Harrison Parker
 * @github https://github.com/harrisoncparker
 */

class WP_Endpoint
{
	private $route;
	private $callable;

	/**
	 * WP_Endpoint constructor.
	 * @param $route
	 * @param $callable
	 */
	public function __construct($route, $callable)
	{
		$this->route = $route;
		$this->callable = $callable;
	}

	/**
	 * @return void
	 */
	public static function add($route, $callable)
	{
		$wpEndpoint = new static($route, $callable);

		$wpEndpoint->addRewrite();
		$wpEndpoint->preventWPFromEvaluatingFalse();
		$wpEndpoint->handleEndpointHit();
	}

	/**
	 * @return void
	 */
	private function addRewrite()
	{
		add_action('init', function () {
			add_rewrite_endpoint($this->route, EP_ROOT);
		});
	}

	/**
	 * @return void
	 */
	private function preventWPFromEvaluatingFalse() 
	{
		add_filter( 'request', function($vars=[]){
			if(isset ( $vars[$this->route] ) && empty ( $vars[$this->route] )){
				$vars[$this->route] = 'default';
			}
			return $vars;
		});
	}

	/**
	 * @return void
	 */
	private function handleEndpointHit() 
	{
		add_action('template_redirect', function () {
			if (get_query_var($this->route)) {
				call_user_func($this->callable);
			};
		});
	}
}