[{Title:"hello",keyPoints:[[{text:"Line 1"},],[{text:"Line 2"},{text:"Line 2 version 2"}],[{nottext:"Line 3"}]]}]

db.collection.aggregate([{$project:{Title:1,keyPoints:{$map:{input:"$keyPoints",in:{$arrayElemAt:["$$this",{$subtract:[{$size:"$$this"},1]}]}}}}}])