TweakTown does note that while the game's minimum requirements list a solid state disk, you can still play the game with a hard disk drive. What is really means is that gamers without an SSD will be frustrated by very long load times. Even with the previous expansion, gamers with HDDs often had to suffer through minutes-long game load times and this is only going to get worse.
So yes, you can still play WoW with an HDD. But there's no guarantee it will be an acceptable experience.
New minimum spec requirements for World of Warcraft Shadowlands explicitly call for an SSD, listings on the Battle.net app confirm. This is the first time any major World of Warcraft expansion has called for an SSD and is an interesting move for Blizzard. Right now game developers are starting to target SSD storage in games thanks to new tech like Microsoft's DirectStorage, Sony's built-in PS5 SSD architecture, and the new RTX IO.