According to https://us.battle.net/support/en/article/286044,
Purchasing the Tavern Pass at any point during the expansion will immediately unlock all the exclusive Tavern Pass rewards up to your current experience level.
The Tavern Pass product lasts for the duration of its expansion. With the launch of a new expansion your Tavern Pass will expire and you can buy the Tavern Pass for the new expansion if you wish.
I tried to purchase the Sunken City Tavern Pass, which is still visible in the client, today, but I saw an error that there are no Tavern Passes available. Nathria launches on August 2nd, which hasn’t happened yet, so I should be allowed to purchase the pass in order to obtain cosmetics. If not, this is false advertising.
