What is the rewards for the 16 anniversary?

[Rhielle-sentinels (Rhielle)]

This was already explained to you.

In case you didnt already know, you can get this quest with the base edition of shadowlands. All you need to do is click the tail of the mount of someone that has bought the heroic/epic edition and you can start the quest. Then click the tail again to hand in the quest.

Here… need more proof? How about the blue post:

@Kaivax

Since the BlizzCon announcement of Shadowlands , we’ve seen concerns about whether it is intended that the quest for the Vestments of the Eternal Traveler transmogrification set can obtained from another player’s Ensorcelled Everwyrm mount.

This is not a bug. Access to this transmogrification set is a bonus available to anyone who purchases Shadowlands . While the Heroic Edition and Epic Edition also grant the Ensorcelled Everwyrm flying mount that summons the questgiver Xolartios, anyone who has purchased Shadowlands can interact with the questgiver and earn the transmog set. The quest for the Vestments of the Eternal Traveler will also remain available to all WoW accounts that have upgraded to Shadowlands after the launch of Shadowlands .

The questgiver does not offer the quest and rewards to player characters who have not yet pre-purchased Shadowlands .

https://us.forums.blizzard.com/en/wow/t/vestments-of-the-eternal-traveler-transmogrification-set/406177

Which also proves that this quest has existed way longer than the anniversary event. You probably just forgot about it and it popped up when you received echoes or logged in.