The Public Test Realm for Wrath of the Lich King Classic Patch 3.4.2 has been updated, but servers are offline currently. Blizzard announced Patch 3.4.2 Testing would begin May 4th. Surprisingly, the client has been updated to Dragonflight, with an entirely new Settings window, which can be seen below —
Updated System Menu on WotLK Classic 3.4.2
For Wrath of the Lich King Classic Phase 1 and Phase 2, the game has been running on a stripped down Shadowlands Client.