Blizzard games no longer use BitTorrent but a proprietary http-based protocol called ngdp. BitTorrent was causing a lot of issues with firewalls so users were disabling it, so they had added http mirrors to them... And then CDNs became a thing, the rest is history.
I'll be happy to give more details on ngdp if you are curious.
They basically created their own git protocol + virtual filesystem, optimized for asset patches inside large compressed binary files. I wish they'd open source it.
I'll be happy to give more details on ngdp if you are curious.