Remove the preview field from SilverStripe

CMSMainExtension.php

<?php

use SilverStripe\Core\Extension;
use SilverStripe\Forms\Form;

class CMSMainExtension extends Extension
{
    /**
     * Removes the CMS preview by removing the literal field which
     * renders the page. This needs to be attached to `CMSMain`:
     *
     * ```
     * SilverStripe\CMS\Controllers\CMSMain:
     *   extensions:
     *     - CMSMainExtension
     * ```
     *
     * For it to work, the `hide_cms_preview` static has to be set.
     * Either add it to non-editable classes in .yml by:
     *
     * ```
     * SilverStripe\CMS\Model\RedirectorPage:
     *   hide_cms_preview: true
     * ```
     *
     * Or to your own classes:
     *
     * ```
     * private static $hide_cms_preview = true;
     * ```
     *
     * Don't forget to run `flush=all`
     *
     * @param Form &$form
     */
    public function updateEditForm(Form &$form)
    {
        $classNameField = $form->Fields()->dataFieldByName('ClassName');

        if (!$classNameField) {
            return false;
        }

        $className = $classNameField->Value();

        if (!$className || !class_exists($className)) {
            return false;
        }

        if (!$className::config()->uninherited('hide_cms_preview')) {
            return false;
        }

        $form->Fields()->removeByName('SilverStripeNavigator');
        $form->removeExtraClass('cms-previewable');
    }
}