public sealed class PointCloudFilterDefinition 

//Must be called on the MCT
//var pcsl = ...;
//Retrieve the available classification codes
//At 2.x - var dict = pcsl.QueryAvailableClassCodesAndLabels();
var dict = pcsl.GetAvailableClassCodesAndLabels();

//Filter out low noise and unclassified (7 and 1 respectively)
//consult https://pro.arcgis.com/en/pro-app/help/data/las-dataset/storing-lidar-data.htm
var filterDef = new PointCloudFilterDefinition()
{
  ClassCodes = dict.Keys.Where(c => c != 7 && c != 1).ToList(),
  ReturnValues = new List<PointCloudReturnType> { 
                         PointCloudReturnType.FirstOfMany }
};
//apply the filter
pcsl.SetFilters(filterDef.ToCIM());