The Team Rocket Balloon, which has been teased in promotional material from Niantic for some time now, is now available. On release, the balloon was available to Lv.40 players...
Mysterious balloons are beginning to appear in Pokémon GO! Trainers who are Level 40 can now encounter Team GO Rocket in balloons. We will continue to update as they appear to more Trainers.— Niantic Support (@NianticHelp) July 7, 2020
...and was later opened to players Lv.35 and over.
Trainers who are Level 35 and above are now able to encounter Team GO Rocket balloons.— Niantic Support (@NianticHelp) July 7, 2020
Currently, it is unknown what the final minimum level will be to encounter the balloon. Beyond that, we also don't know all of the specifics of this feature yet. Thus far, it is known that the Team Rocket Balloon appears on the map and can be interacted with to initiate a single Team Rocket battle.
Some players have reported that they need to zoom out the camera to see/interact with the balloon, and it is confirmed that players will receive Team Rocket Radar Pieces from these battles.