Source: includes/ajax/class-pages.php

<?php
namespace um\ajax;

use WP_Query;

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

/**
 * Class Pages
 *
 * @package um\ajax
 */
class Pages {

	/**
	 * Pages constructor.
	 */
	public function __construct() {
		add_action( 'wp_ajax_um_get_pages_list', array( $this, 'get_pages_list' ) );
	}

	/**
	 * AJAX callback for getting the pages list
	 */
	public function get_pages_list() {
		check_ajax_referer( 'um-admin-nonce', 'nonce' );

		// we will pass post IDs and titles to this array
		$return = array();

		$pre_result = apply_filters( 'um_admin_settings_get_pages_list', false );

		if ( false === $pre_result ) {
			$query_args = array(
				'post_type'           => 'page',
				'post_status'         => 'publish', // if you don't want drafts to be returned
				'ignore_sticky_posts' => 1,
				'posts_per_page'      => 10, // how much to show at once
				'paged'               => ! empty( $_GET['page'] ) ? absint( $_GET['page'] ) : 1,
				'orderby'             => 'title',
				'order'               => 'asc',
			);

			if ( ! empty( $_GET['search'] ) ) {
				$query_args['s'] = sanitize_text_field( $_GET['search'] ); // the search query
			}

			$field_id = ! empty( $_GET['field_id'] ) ? sanitize_text_field( $_GET['field_id'] ) : null;
			if ( 'form__um_register_use_gdpr_content_id' === $field_id ) {
				$predefined_ids   = array();
				$predefined_pages = array_keys( UM()->config()->get( 'predefined_pages' ) );
				foreach ( $predefined_pages as $slug ) {
					$p_id = um_get_predefined_page_id( $slug );
					if ( empty( $p_id ) ) {
						continue;
					}
					$predefined_ids[] = $p_id;
				}
				$predefined_ids = array_unique( $predefined_ids );
				if ( ! empty( $predefined_ids ) ) {
					$query_args['post__not_in'] = $predefined_ids;
				}
			}

			/**
			 * Filters WP_Query arguments for getting pages visible in the dropdown fields in UM Settings.
			 *
			 * @since 2.10.6
			 * @hook  um_admin_settings_get_pages_list_args
			 *
			 * @param {array}  $query_args Get pages WP_Query arguments.
			 * @param {string} $field_id   Dropdown field ID.
			 *
			 * @return {array} Get pages WP_Query arguments.
			 */
			$query_args = apply_filters( 'um_admin_settings_get_pages_list_args', $query_args, $field_id );

			$search_results = new WP_Query( $query_args );

			if ( $search_results->have_posts() ) {
				while ( $search_results->have_posts() ) {
					$search_results->the_post();

					// shorten the title a little
					$title    = ( mb_strlen( $search_results->post->post_title ) > 50 ) ? mb_substr( $search_results->post->post_title, 0, 49 ) . '...' : $search_results->post->post_title;
					$title    = sprintf( __( '%s (ID: %s)', 'ultimate-member' ), $title, $search_results->post->ID );
					$return[] = array( $search_results->post->ID, $title ); // array( Post ID, Post Title )
				}
			}

			$return['total_count'] = $search_results->found_posts;
		} else {
			// got already calculated posts array from 3rd-party integrations (e.g. WPML, Polylang)
			$return = $pre_result;
		}

		wp_send_json( $return );
	}
}