SERVER-78260 was a bug related to "lifting" negation predicates outside of an $elemMatch. We appear to have solved this bug by making it so that $not under object $elemMatch cannot use an index, for example:

> db.foo.createIndex({"arr.b": 1})
> db.foo.find({arr: {$elemMatch: {b: {$ne: 1}}}}).explain()
// collscan

It seems to me like this query could use this index (with a residual filter), the same way that the non-negated query can.
 
This ticket is to allow $not within an object $elemMatch to be able to use an index on the nested field. However, we should make sure not to re-introduce the bug fixed by SERVER-78260.