Niantic has announced that a new feature called Collection Challenges has been added to the game to start the new year 2021.

Collection Challenges are here!

Trainers, A new year means new challenges. And in that spirit, we’re introducing a new feature—Collection Challenges!

Whether you’re new to Pokémon GO or a seasoned Trainer, Collection Challenges provide a handy guide to collecting Pokémon featured during certain events. Different in-game events will feature different Collection Challenges, helping you grow into the best Trainer you can be.

To get started on a Collection Challenge, open the Today View to see which Pokémon are part of that challenge. Each challenge will disappear once its corresponding in-game event ends, so do your best to collect all the Pokémon in the challenge before then! As you complete Collection Challenges featured in different in-game events, you’ll be able to earn rewards and add to your new Elite Collector medal!

The first Collection Challenge is rolling out now! Complete the Unova-themed Collection Challenge during the event by catching Snivy, Tepig, Oshawott, Lillipup, Herdier, Roggenrola, Solosis, Ferroseed, and Blitzle, to receive a reward of Stardust, 25 Poke Balls, and 5 Rare Candy. We look forward to seeing you complete your Collection Challenges!

Source: https://pokemongolive.com/post/collections-launch/?hl=en