Loading custom page template via plugin

Question

I am following a rather outdated tutorial on custom post types and the last step is to create a custom page template, where this is intended to show all posts of the custom type in archive listing format. As per the comments, the methods used no longer work 6 years later (go figure) and my custom page template isn't an option when creating a new page.

If I place my custom template in the theme folder it works but the option to load it from within the plugin directory has broken. How can I fix this so I can assign the template to a new page?

Edit

As I better understand now, this code is only loading the template when viewing my custom post types. The confusion was due to it showing as an available template on the "Page Attributes" section when it was located in the Themes directory. I am wanting to have it available as an option when making a new page, but have the template file located in my plugin's directory.

The original code from the tutorial:

add_filter( 'template_include', 'include_template_function', 1 );

function include_template_function( $template_path ) {
    if ( get_post_type() == 'movie_reviews' ) {
        if ( is_single() ) {
            // checks if the file exists in the theme first,
            // otherwise serve the file from the plugin
            if ( $theme_file = locate_template( array ( 'single-movie_reviews.php' ) ) ) {
                $template_path = $theme_file;
            } else {
                $template_path = plugin_dir_path( __FILE__ ) . '/single-movie_reviews.php';
            }
        }
    }
    return $template_path;
}

Answer 1

To add custom template in page attributes template section you have to first add your template to dropdown and load it in template_include hook when current page has selected it as current template.

/**
 * Add "Custom" template to page attirbute template section.
 */
function wpse_288589_add_template_to_select( $post_templates, $wp_theme, $post, $post_type ) {

    // Add custom template named template-custom.php to select dropdown 
    $post_templates['template-custom.php'] = __('Custom');

    return $post_templates;
}

add_filter( 'theme_page_templates', 'wpse_288589_add_template_to_select', 10, 4 );


/**
 * Check if current page has our custom template. Try to load
 * template from theme directory and if not exist load it 
 * from root plugin directory.
 */
function wpse_288589_load_plugin_template( $template ) {

    if(  get_page_template_slug() === 'template-custom.php' ) {

        if ( $theme_file = locate_template( array( 'template-custom.php' ) ) ) {
            $template = $theme_file;
        } else {
            $template = plugin_dir_path( __FILE__ ) . 'template-custom.php';
        }
    }

    if($template == '') {
        throw new \Exception('No template found');
    }

    return $template;
}

add_filter( 'template_include', 'wpse_288589_load_plugin_template' );

theme_page_templates hook is available for page post type. If you want to add custom template to other post type you must replace page with your custom post type name e.g. event post type hook will have a name theme_event_templates.