[{"_id":"1","key":"a","language":"en","version":"0.1",},{"_id":"2","key":"b","language":"en","version":"0.1",},{"_id":"3","key":"b","language":"en","version":"0.2",},{"_id":"4","key":"c","language":"en","version":"0.2",}]

db.collection.aggregate([{"$sort":{"version":1}},{$group:{"_id":"$key","items":{"$push":"$$ROOT"}}},{"$match":{"items.version":{"$ne":"0.1"}}},{"$replaceRoot":{"newRoot":{"$last":"$items"}}}])