Such a feature would allow handling property paths with one or more collection-valued segments in it, i.e. collection-valued navigation or complex properties.

There are use cases in data aggregation that require to deal with such paths, e.g. to group by properties that can only be reached via a path involving a collection-valued navigation property like this:

GET ~/Products?$apply=groupby((Name,Sales/Customer/ID), aggregate(TaxRate with average as AverageTaxRate))

ODATA-1206 declared support for such paths with collection-valued properties in groupby, however it lacks a description how to process them and furthermore ODATA-1404 withdraws this statement by restricting grouping properties to be reached via single-valued navigation and complex properties.