Dear David Rock,
The FilterRule is applicable to element parameter related, so the FilterRule can be retrieved from ElementParameterFilter.
Note that ElementParameterFilter is derived class of ElementFilter, so you can cast the ParameterFilterElement.GetElementFilter() to ElementParameterFilter first, and then retrieve the filter rules via ElementParameterFilter.GetRules().
The better API documentation should like this: "This method is deprecated in Revit 2019 and will be removed in the next version of Revit. We suggest you instead use GetElementFilter instead: cast the filter to ElementParameterFilter and then retrieve the filter rules from ElementParameterFilter.GetRules()."
Jim Jia
Autodesk Forge Evangelist
https://forge.autodesk.com
Developer Technical Services
Autodesk Developer Network
Email: Jim.Jia@autodesk.com