It's a new "feature", or rather it's a feature that got removed. Whistling no longer attracts the nearest horse from what I can tell. Apparently it is due to repeated calls for cavalry nerf, some people felt it was unfair for riders to get free extra "lives" by calling on spare horses running around the battlefield.
This is mostly from hearsay, it could just a patch fucking something up like with the jumpshot being back in for archers, something that was never intended.
If you are cav and one of your teammates gets dehorsed, fuck up an enemy cav and stop the horse physically, just like before whistling.