[{"_id":1,"url":"a","status":"200"},{"_id":2,"url":"a","status":"400"},{"_id":3,"url":"b","status":"400"},{"_id":4,"url":"c","status":"400"},{"_id":5,"url":"c","status":"400"}]

db.collection.aggregate([/** GROUP BY URL*/{$group:{_id:"$url",/** $$ROOT STORE IN ROOT*/root:{$first:"$$ROOT"},count:{$sum:1}}},/** MATCH UNIQUE URL*/{$match:{count:{$eq:1}}},/** ABOVE FILTERED DATA REPLACE IN ROOT*/{$replaceRoot:{newRoot:"$root"}},/** MATCH CONDITION FOR 400*/{$match:{status:"400"}}])