Source

File: wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php

public function delete_item_permissions_check( $request ) {
		$user = $this->get_user( $request['id'] );
		if ( is_wp_error( $user ) ) {
			return $user;
		}

		if ( ! current_user_can( 'delete_user', $user->ID ) ) {
			return new WP_Error(
				'rest_user_cannot_delete',
				__( 'Sorry, you are not allowed to delete this user.' ),
				array( 'status' => rest_authorization_required_code() )
			);
		}

		return true;
	}