Plugin to add the page template to the Pages admin

show-templates.php

<?php
/**
 * Show Page Templates
 *
 * @package     show-page-templates
 * @author      ryanshoover
 * @license     Proprietary
 *
 * @wordpress-plugin
 * Plugin Name: Show Page Templates
 * Plugin URI:  wordpress.org
 * Description: Shows page templates filterable
 * Version:     0.1.0
 * Author:      ryanshoover
 * Author URI:  ryan.hoover.ws
 * Text Domain: show-page-templates
 * License:     Proprietary
 */

namespace ShowPageTemplates;

function register( $columns ) {
	$columns[ 'page-template'] = __( 'Template', 'show-page-templates' );

	return $columns;
}

add_filter( 'manage_page_posts_columns', __NAMESPACE__ . '\register' );

function sortable( $columns ) {
	$columns['page-template'] = 'page-template';

	return $columns;
}

add_filter( 'manage_edit-page_sortable_columns', __NAMESPACE__ . '\sortable' );

function filter_dropdown(){
	if ( empty( $_GET['post_type'] ) || 'page' !== $_GET['post_type'] ) {
		return;
	}

	$templates = array_flip( get_page_templates( null, 'page' ) );

	$selected_value = ! empty( $_GET['page-template'] ) ? esc_attr( $_GET['page-template'] ) : '';

	?>
	<select name="page-template">
		<option value="">All Templates</option>
		<?php
		foreach ( $templates as $value => $name ) :
			?>
			<option
				value="<?php esc_attr_e( $value ); ?>"
				<?php selected( $value, $selected_value, true ); ?>
			>
				<?php esc_html_e( $name ); ?>
			</option>
			<?php
		endforeach;
		?>
	</select>
	<?php
}

add_action( 'restrict_manage_posts', __NAMESPACE__ . '\filter_dropdown' );

function content( $column, $post_id ) {
	static $templates;

	$templates = $templates ?: array_flip( get_page_templates( null, 'page' ) );

	if ( 'page-template' === $column ) {
		$template = get_page_template_slug( $post_id );

		$template_name = ! empty( $templates[ $template ] ) ? $templates[ $template ] : 'Default';

		esc_html_e( $template_name, 'show-page-templates' );
	}

	return $column;
}

add_filter( 'manage_pages_custom_column', __NAMESPACE__ . '\content', 10, 2 );

function get_posts( $query ) {
	if ( ! is_admin() ) {
		return;
	}

	if ( 'page-template' == $query->get( 'orderby' ) ) {
		$query->set( 'meta_key', '_wp_page_template' );
		$query->set( 'orderby', 'meta_value' );
	}

	if ( ! empty( $_GET['page-template'] ) ) {
		$meta_query = $query->get( 'meta_query' );

		$meta_query = $meta_query ?: [];

		$meta_query[] = [
			'key'   => '_wp_page_template',
			'value' => esc_attr( $_GET['page-template'] ),
		];

		$query->set( 'meta_query', $meta_query );
	}
}

add_action( 'pre_get_posts', __NAMESPACE__ . '\get_posts' );