EDIT: Yesterday, september 1st, Blizzard changed the Shadowlands minimum requirements. Now it lists HDD in the minimum requirements. but says it can impact player experience and an SSD is still recommended. Bottom line - if you can play BFA you can probably play Shadowlands on the same level of performance. Original OP below.
Just a reminder to buy an SSD if you intend on playing Shadowlands. WoW is already very annoying to play on a normal HDD, after 8.1.5 changed how assets are being loaded, you can spend 2 minutes loading into Dalaran on the Broken Shore and I’m sure Shadowlands will be even worse.
System requirements going up with time is normal for WoW, and designing the game around SSDs can open up more interesting design options in the future. So just go get an SSD if you haven’t already.