In OData V1, when we originally defined query options for filter, ordering, expand, etc., we prefixed them with "$" so as not to conflict with "custom" query options that a service might implement.

In reality, it is highly unlikely that an OData-compliant service would need to implement a query option named "filter" that is different than "$filter", or "expand" that is different than "$expand", etc.

At the same time, we've been getting pushback that services don't want to have query options prefixed with "$". In fact, this has been an adoption blocker in cases, especially since it apparently raises issues with some tools.

Prefixing query options with "$" just creates one more difference between how customers want to implement a service and how they have to implement the service in order to be OData-compliant.