tldr version: not in active development, not even a final concept, just a bunch of ideas.
You can find precursors in the WvW rank boxes, but that's not the scavenger hunt.
And here are the full quotes:
We still plan to do both new legendary weapons, as well as a clear path (on top of the current random chance) to gain precursors which players can see their progress and understand how much work they have left to do to gain it.
It’s highly likely the precursor concept will involve playing varied different content all over the game to gather numerous content specific materials that are combined together to make a precursor.
We do also want to maintain that chance you could get lucky and earn one earlier through random drops, so when we add the new system we will not remove the current one to ensure players have multiple ways to gain the items. Also with the upcoming update to WvW, there will be a chance players can earn precursors from the WvW rank reward chests as well.
We’ve learned we really can’t give timelines, since if we’re ever wrong a mob comes after us with torches and pitchforks I can say we started designing this, and realized the proper (not hacked in quick) solution needs to be a part of bigger systems that more clearly directs players as well as ties in comprehensive rewards for playing across our game world, which we’re currently working on right now.
Some small parts of that comprehensive plan for game reward systems are already rolling out this month with the updates to WvW rank rewards.
For more details of our high-level strategy, feel free to check out: