There are discussions below regarding how to create a singleton that allows subclassing. It seems with get_called_class  there is now a cleaner solution than what is discussed below, that does not require overriding a method per subclass.

e.g.

<?php
abstract class MySuperclass {
    static private $instances = array();

    static public function getInstance(): ACFBlock {
        $className = get_called_class();
        if (!isset(self::$instances[$className])) {
            self::$instances[$className] = new static();
        }
        return self::$instances[$className];
    }

    private function __construct() {
        // Singleton
     }
}

class MySubclass extends MySuperclass {
}

MySubclass::getInstance();