[{name:"Bob",a:{02910143:{val:10},101B70BB:{val:20},7A6C86F1:{val:10},}}]

db.collection.aggregate([{"$addFields":{"a":{"$objectToArray":"$a"}}},{"$addFields":{"a":{"$filter":{"input":"$a","as":"a","cond":{$ne:["$$a.v.val",10]}}}}},{"$addFields":{"a":{"$arrayToObject":"$a"}}}])