Restrict hierarchical taxonomy term relation to a certain level.

wp-taxonomy-term-parent-child-relation.php

<?php
/**
 * Restrict category term relation to only parent-child
 *
 * @see https://wpseek.com/hook/post_edit_category_parent_dropdown_args/
 * @see https://wpseek.com/hook/taxonomy_parent_dropdown_args//
 * 
 * @param array $args
 * @return array Modified arguments
 */
function op_restrict_taxonomy_term_relation( $args ) {
	if ( 'category' === $args['taxonomy'] ) {
		$args['depth'] = 1;
	}
	return $args;
}
add_filter( 'post_edit_category_parent_dropdown_args', 'op_restrict_taxonomy_term_relation' );
add_filter( 'taxonomy_parent_dropdown_args', 'op_restrict_taxonomy_term_relation' );

wp-taxonomy-term-restricted-relation.php

<?php
/**
 * Restrict hierarchical taxonomy term relation to certain level
 * 
 * For instance:
 * 1 for parent - child relation
 * 2 for grand parent - parent - child / grand child relation
 *
 * @see https://wpseek.com/hook/post_edit_category_parent_dropdown_args/
 * @see https://wpseek.com/hook/taxonomy_parent_dropdown_args//
 * 
 * @param array $args
 * @return array Modified arguments
 */
function op_restrict_taxonomy_term_relation( $args ) {
	/**
	 * Taxonomy name to compare with
	 * or which taxonomy relation you wanna change
	 * 
	 * @var string $taxonomy
	 */
	$taxonomy = '';

	/**
	 * Taxonomy relation depth
	 * 
	 * @var int $depth
	 */
	$depth = 0;

	if ( $taxonomy === $args['taxonomy'] ) {
		$args['depth'] = $depth;
	}
	return $args;
}

add_filter( 'post_edit_category_parent_dropdown_args', 'op_restrict_taxonomy_term_relation' );
add_filter( 'taxonomy_parent_dropdown_args', 'op_restrict_taxonomy_term_relation' );