I am afraid that the information that your require is not available at the moment the user deletes a filter.
But even if it was, how is our data engine going to handle a filter operator which it does not support? In other words, what will be written in the respective FilterDescriptor (Member-Operator-Value) before it is used for filtering by the data engine. The data engine builds a LINQ query based on the stuff written in Member-Operator-Value, for example:
var result = sourceCollection.Where(customer => customer.Name == "John");
The data engine supports only a finite set of FilterOperator's and know what LINQ queries to compose when it encounters one of them.
How are your custom operators going to be supported, i.e. who is going to create the approapriate LINQ query?
the Telerik team