[{"year":2021},{"year":2021},{"year":2020},{"year":2020},{"year":2019},{"year":2019},{"year":2018},{"year":2017}]

db.collection.aggregate({$group:{_id:"$year",movies:{$sum:1}}},{"$group":{"_id":"$movies","yearList":{"$push":"$$ROOT._id"}}},{$sort:{"_id":-1}},{"$limit":1})