Blizzard have clarified how the end of Shadowlands will affect the Great Vault and the timing of when players can claim their final gear from it.
Great Vault (Source)
The week of November 22 will be the last week that new loot options will become available in the Shadowlands Great Vault. Players will be able to collect their final reward, including any items from previously unclaimed weeks, until Monday November 28 at 3 p.m. PST. After this time, any remaining loot will be lost.
We’ll see you in Dragonflight!