D7 domain.api.php contains D6 refs

domain_prefix is removed from the 7.x branch

I'm not sure a replacement is designed so no patch

/**
 * Allows modules to alter path when rewriting URLs.
 *
 * This hook will fire for all paths and may be resource-intensive.
 * Look at Domain Prefix for best practices implementation. In Domain
 * Prefix, we only include this function if we know it is necessary.
 *
 * @see domain_prefix_init()
 * @see hook_url_outbound_alter()
 *
...
 *
 * @ingroup domain_hooks
 */
function hook_domain_path($domain_id, &$path, &$options, $original_path) {

This is not removed by activities through #777230: Hook Changes (d7 cleanup)