This was an intentional (and breaking) change made to address a major issue with filtering. $orderby is still very much a thing.
To sum up from that link, if you use both $orderby and $filter in a request:
Any fields in $orderby MUST also be in $filter.
Order of fields in $filter matters:
Fields that are also in $orderby MUST come first in the $filter and MUST be in the same order.
Fields that are not in $orderby MUST come after the fields that are in $orderby.
So according to those guidelines, the problem with your request is that flag/dueDateTime/dateTime is not present in $filter.
