Blizzard used to have p2p support, they removed it around 2015. It’s not hard to think of a bunch of problematic cases which become absolute hell to diagnose because they’re client side.
Their downloaders for classic games still have the options to enable peer to peer. Though it failed to initialise, but I'm not sure if that's because their tracker is down or because it demands upnp. I recently did this with Diablo 2 and it's expansion.