Icecast versions 1.3.0 through 1.3.12 would reply with "ICY 200"
under certain conditions:

    client_wants_icy_headers (connection_t *con)
    {
            const char *val;

            if (!con)
                    return 1;

            val = get_user_agent (con);
            if (!val || !val[0] || strcmp (val, "(null)") == 0)
                    return 1;

            if (con->food.client->use_icy)
                    return 1;
            if (strncasecmp (val, "winamp", 6) == 0)
                    return 1;
            if (strncasecmp (val, "Shoutcast", 9) == 0)
                    return 1;

            return 0;
    }

So mainly if there is no 'user agent' or it is '(null)' or contains
'winamp' or 'Shoutcast'.

No mainstream distribution carries Icecast 1.3.x anymore, after all
it was released in 2002 and superseded by Icecast 2.x.