Example module to modify/remove page "addable" permission for pages with level greater than one and basic-page template

PageAddableHook.module

<?php

/**
 * ProcessWire example demonstration module
 *
 * Page::addable hook module, once installed will set permission "addable" to false
 * for pages using basic-page template and with level greater than 1
 */

class PageAddableHook extends WireData implements Module, ConfigurableModule {

    /**
     * getModuleInfo is a module required by all modules to tell ProcessWire about them
     *
     * @return array
     *
     */
    public static function getModuleInfo() {

        return array(
            'title' => 'PageAddableHook',
            'version' => 1,
            'summary' => 'An example module on how to hook into Page::addable page permission hook.',
            'href' => 'http://www.processwire.com',
            'singular' => true,
            'autoload' => true,
            );
    }

    /**
     * Initialize the module
     *
     * ProcessWire calls this when the module is loaded. For 'autoload' modules, this will be called
     * when ProcessWire's API is ready. As a result, this is a good place to attach hooks.
     *
     */
    public function init() {
        
        // hook into page permissions
        // note: Page::addable is also added via hook in wire/modules/PagePermissions.module
        // that's why you won't find those in wire/core/Page.php
        $this->addHookAfter("Page::addable", $this, "hookPageAddable");
        
    }


    public function hookPageAddable($event){

        // already don't have add childs permission so we don't need to do anything
        if(!$event->return) return;

        // get the page, the class in this case is the Page
        $page = $event->object;

        if($page->template == "basic-page"){

            // only allow 2 levels of basic-page
            if($page->parents->count() > 1){

                // return false to remove "addable" permission
                $event->return = false;
            }

        }

    }

}