Although there is the important Level Squish with the new expansion of World of Warcraft, the first 20 free trial levels will still remain absolutely active even after the arrival of Shadowlands, thus guaranteeing all players access to the special reward protagonist of this news.
So, no fear, even in 2021 we will be able to make our alternative heroine of the Paladin class simply by reaching level 20 with a character on WoW.
Here is the official post of Celestalon, and the one related to the announcement of the Lady Liadrin.
@Celestalon got a quick question that I don't see an answer to yet (maybe my googling skills are shit) – how will obtaining Liadrin skin in HS change with Shadowlands lv squish next week?
— rikapi (@baratie) October 5, 2020