Links in the documentation to $arrayToObject point to the following URL:

https://docs.mongodb.com/manual/reference/operator/aggregation/arrayToObject/

When you go to this link, the server returns a 301 Moved Permanently to:

https://docs.mongodb.com/manual/reference/operator/aggregation

The documentation for $arrayToObject does exist:

https://github.com/mongodb/docs/blob/master/source/reference/operator/aggregation/arrayToObject.txt