Blizzard has updated their minimal required spec to add 100GB storage area for both SSDs and hard drives. Howeverthey warn players who functionality of your hard drive may influence Shadowland's gaming experience.
A couple of Shadowlands beta testers have reported that running the game in an HDD hasn't affected their gameplay or performance in any significant way.
However, it is most likely a great idea to run this game on an SSD anyway (here's our list of Greatest SSDs). If you have a look at games such as Call of Duty Modern Warfare which don't require an SSD but have very large storage requirements, the game is playable but you will encounter painfully long loading times. It is also possible to possibly experience"hitching" in which the HDD is not fast enough to stream assets in real time.