WordPress at a glance
function is not described

allow_subdomain_install() WP 3.0.0

Allow subdomain installation

No Hooks.

Return

true/false. Whether subdomain installation is allowed

Usage

allow_subdomain_install();

Code of allow subdomain install: wp-admin/includes/network.php VER 5.0.3

<?php
function allow_subdomain_install() {
	$domain = preg_replace( '|https?://([^/]+)|', '$1', get_option( 'home' ) );
	if ( parse_url( get_option( 'home' ), PHP_URL_PATH ) || 'localhost' == $domain || preg_match( '|^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$|', $domain ) )
		return false;

	return true;
}