db={"collection":[{"_id":ObjectId("656e59214247fdc6cb90136d"),"title":"Caught","year":1900,"cast":["Undefined"],"genres":["Undefined"]},{"_id":ObjectId("656e59214247fdc6cb90136e"),"title":"Caught2","year":1920,"cast":["Undefined"],"genres":["Undefined"]},{"_id":ObjectId("656e59214247fdc6cb90136f"),"title":"Caught3","year":1940,"cast":["Undefined"],"genres":["Undefined"]}]}

db.collection.aggregate([{"$group":{"_id":null,"maxYear":{"$max":"$year"}}},{"$addFields":{"minYear":{"$subtract":["$maxYear",20]}}},{"$lookup":{"from":"collection","let":{"myMinYear":"$minYear"},"as":"docs","pipeline":[{"$match":{"$expr":{"$gte":["$year","$$myMinYear"]}}}]}}])