Updated: Feb 24, 2020
Blizzard has just pushed a new game product - the WoW Classic PTR.
It is unclear how this will be used as there's been no official statement by Blizzard, but perhaps it could be used to test raids in later phases like Naxxramas--or if Blizzard ever deviates from #nochanges, to try out game tweaks. It's notable that this game product popped up now and not sooner, after Phase 3 Blackwing Lair launched. Blizzard has also been open to the idea of Burning Crusade and Wrath Legacy servers, so if they do come to pass, testing could occur on this PTR as well.