The old signature for the IsFieldNeeded method only took in a const char* for the name of the field and that was deprecated since it wasn’t descriptive enough. The new, replaced method is actually still called IsFieldNeeded but now also takes in a type which corresponds to the vtkDataObject::AttributeTypes enum. The basic idea here is that a simulation could provide the same field name in both a point data field and a cell data field and in the old IsFieldNeeded form there was no way of knowing which one the adaptor should provide.
If you do a search through the examples under the Examples/Catalyst directory of the main ParaView source code for IsFieldNeeded you’ll see some examples.