https://www.notebookcheck.net/Blizzard-You-will-need-an-SSD-to-play-World-of-Warcraft-Shadowlands.490876.0.html
According to multiple user reports that were shared on the official forum, the game simply loads much faster when installed on an SSD, but should work just fine on a hard drive for those who have a bit of patience. Remains to see if Blizzard will change the list of minimum system requirements accordingly before the October 27 global launch of World of Warcraft: Shadowlands.
You should still get an SSD because why punish yourself over using a spinnig disk in 2020, but it seems it will still work on the ancient hard drive.