I used to be involved with this sort of thing, and it's actually pretty interesting stuff. There are enormous open source projects, operating largely out of public sight, that have recreated the WoW server backend in most of its glory. The lower-level content works quite well these days, though a lot of complex raid scripts are missing or broken.
It's pretty rare for Blizzard to go after these companies in court, but it's happened here and there. Look up "Blizzard Entertainment, Inc. v. Alyson Reeves" if you're curious what such a lawsuit looks like! Every patch since Lich King, though, has attacked these reverse engineers on the technical front by adding layers upon layers of obfuscation to the protocol. That's had some effect, and many private servers have stuck with the old Burning Crusade or Lich King releases.
This one is probably the most popular: https://github.com/TrinityCore/TrinityCore
It's pretty rare for Blizzard to go after these companies in court, but it's happened here and there. Look up "Blizzard Entertainment, Inc. v. Alyson Reeves" if you're curious what such a lawsuit looks like! Every patch since Lich King, though, has attacked these reverse engineers on the technical front by adding layers upon layers of obfuscation to the protocol. That's had some effect, and many private servers have stuck with the old Burning Crusade or Lich King releases.