in order to get shadowlands you need to already have BFA. So you will not be able to preorder it now.
Edit:
requirements to get the per-purchase
*World of Warcraft®: Shadowlands expansion available on or before December 31, 2020
In-game items and Character Boost not available in World of Warcraft® Classic, and Character Boost usable only on the WoW game account on which it was purchased or redeemed as a gift
Upgrading to the Epic Edition will not re-grant any in-game items previously unlocked on your account
Requires World of Warcraft®: Battle for Azeroth®, Internet connection, Blizzard® registration, and Blizzard® Battle.net® desktop application to play.
To answer this, it’ll be some time before BfA is rolled into the base game. Mind you, it doesn’t roll into the base game the moment SL drops, so about a few weeks.
Just to confirm what others have already - in order to purchase Shadowlands right now, your account must already own Battle for Azeroth. So you won’t be able to pre-order if you aren’t already upgraded*.
*Unless of course you purchase the complete collection which includes Battle for Azeroth.
This is true. Some expansions have rolled into the base game around the time of the pre-patch, so a few weeks before the drop date. Other times, they’ve rolled it in after the drop date. I haven’t seen any announcement yet as to when they will be rolling BfA into the base game.