Adds /all/ URL after a post type archive to be able to show all items for that archive.

all-post-type-archives.php

<?php
/**
 * Plugin Name:  Show All Post Type Archives
 * Description:  Add support for /all/ after a post type archive
 * Version:      0.0.1
* Plugin URI:    https://gist.github.com/kingkool68/efa01ecff91bc722cd3f667ae509a37b
 * Author:       Russell Heimlich
 * Author URI:   https://github.com/kingkool68
 */
class RH_Show_All_Archives {

	/**
	 * The query var we use to determine if the request is an "all" post type archive
	 *
	 * @var string
	 */
	public static $query_var = 'show_all_archive';

	/**
	 * Get an instance of this class
	 */
	public static function get_instance() {
		static $instance = null;
		if ( null === $instance ) {
			$instance = new static();
			$instance->setup_actions();
			$instance->setup_filters();
		}
		return $instance;
	}

	/**
	 * Hook into WordPress via actions
	 */
	public function setup_actions() {
		add_action( 'pre_get_posts', array( $this, 'action_pre_get_posts' ) );
	}

	/**
	 * Hook into WordPress via filters
	 */
	public function setup_filters() {
		add_filter( 'register_post_type_args', array( $this, 'filter_register_post_type_args' ), 10, 2 );
		add_filter( 'query_vars', array( $this, 'filter_query_vars' ) );
		add_filter( 'generate_rewrite_rules', array( $this, 'filter_generate_rewrite_rules' ) );
		add_filter( 'template_include', array( $this, 'filter_template_include' ) );
	}

	/**
	 * Modify the main loop on the "Show All" post type archive page to show all of the posts in the query
	 *
	 * @param  WP_Query $the_query The query to modify
	 */
	public function action_pre_get_posts( $the_query ) {
		if ( ! $the_query->is_main_query() || ! static::is_show_all_archive() ) {
			return;
		}
		$the_query->set( 'posts_per_page', 999 );
	}

	/**
	 * Add the "all-archive" support to a pre-determined set of post types
	 *
	 * @param  array  $args      Register post type arguments to modify
	 * @param  string $post_type The post type being modified
	 */
	public function filter_register_post_type_args( $args = array(), $post_type = '' ) {
		$allowed_post_types = array( 'gallery' );
		if ( in_array( $post_type, $allowed_post_types, true ) ) {
			$args['supports'][] = 'all-archive';
		}
		return $args;
	}

	/**
	 * Register our query var with WordPress
	 *
	 * @param  array $query_vars The list of query vars to modify
	 */
	public function filter_query_vars( $query_vars = array() ) {
		$query_vars[] = static::$query_var;
		return $query_vars;
	}

	/**
	 * Add a /all/ rewrite rule to each post type archive that supports 'all-archive'
	 *
	 * @param  WP_Rewrite $wp_rewrite The WordPress Rewrite object to modify
	 */
	public function filter_generate_rewrite_rules( $wp_rewrite ) {
		$new_rules       = array();
		$post_type_names = static::get_supported_post_types();
		foreach ( $post_type_names as $post_type_name ) {
			$post_type_obj     = get_post_type_object( $post_type_name );
			$the_slug          = $post_type_obj->rewrite['slug'];
			$key               = $the_slug . '/all/?$';
			$value             = 'index.php?post_type=' . $post_type_name . '&' . static::$query_var . '=1';
			$new_rules[ $key ] = $value;
		}
		$wp_rewrite->rules = $new_rules + $wp_rewrite->rules;
	}

	/**
	 * Add support for loading different templates for the /all/ archive
	 *
	 * @param  string $orig_template The originally requested template path
	 */
	public function filter_template_include( $orig_template = '' ) {
		if ( static::is_show_all_archive() ) {
			$post_type      = get_post_type();
			$template_names = array(
				'archive-' . $post_type . '-all.php',
				'archive-all.php',
				'archive-' . $post_type . '.php',
				'archive.php',
				'index.php',
			);
			$new_template   = locate_template( $template_names );
			if ( ! empty( $new_template ) ) {
				return $new_template;
			}
		}
		return $orig_template;
	}

	/**
	 * Get all of the post types that support the 'all-archive' feature
	 */
	public static function get_supported_post_types() {
		$post_types = get_post_types_by_support( 'all-archive' );
		return apply_filters( 'rh_show_all_archives/post_types', $post_types );
	}

	/**
	 * Helper to determine if request is for an "all" post type archive
	 *
	 * @return boolean
	 */
	public static function is_show_all_archive() {
		return ( get_query_var( static::$query_var ) );
	}
}
RH_Show_All_Archives::get_instance();