depth-restriction.php

<?php
/**
 * Show only top level district or division
 * that means district without any parent district
 *
 * @param array $args
 * @return array $args
 */
function op_page_attributes_dropdown_pages_args( $args ) {
	if ( 'district' === $args['post_type'] ) {
		$args['depth'] = 1;
	}
	return $args;
}
add_filter( 'page_attributes_dropdown_pages_args', 'op_page_attributes_dropdown_pages_args' );
add_filter( 'quick_edit_dropdown_pages_args', 'op_page_attributes_dropdown_pages_args' );

district-cpt.php

<?php
// Register Custom Post Type
function op_register_post_type() {
	$labels = array(
		'name'                  => _x( 'Districts', 'Post Type General Name', 'khutkhut' ),
		'singular_name'         => _x( 'District', 'Post Type Singular Name', 'khutkhut' ),
	);
	$args = array(
		'labels'                => $labels,
		'supports'              => array( 'title', 'editor', 'thumbnail', 'page-attributes' ),
		'hierarchical'          => true,
		'public'                => true,
		'menu_icon'             => 'dashicons-location',
	);
	register_post_type( 'district', $args );
}
add_action( 'init', 'op_register_post_type', 0 );

snippet-from-wpcore.php

<?php
// ..... lots of things are here
$dropdown_args = array(
	'post_type'        => $post->post_type,
	'exclude_tree'     => $post->ID,
	'selected'         => $post->post_parent,
	'name'             => 'parent_id',
	'show_option_none' => __('(no parent)'),
	'sort_column'      => 'menu_order, post_title',
	'echo'             => 0,
);

/**
 * Filters the arguments used to generate a Pages drop-down element.
 *
 * @since 3.3.0
 *
 * @see wp_dropdown_pages()
 *
 * @param array   $dropdown_args Array of arguments used to generate the pages drop-down.
 * @param WP_Post $post          The current post.
 */
$dropdown_args = apply_filters( 'page_attributes_dropdown_pages_args', $dropdown_args, $post );
$pages = wp_dropdown_pages( $dropdown_args );
// ..... lots of things are here