The below code iterates over all the feature symbology's used by current DGN file and finds the missing default or 3D template.

//Required References
using Bentley.CifNET.ContentManagementModel;
using Bentley.CifNET.GeometryModel.ContentManagement;
using System.Diagnostics;

public void ReportMissingElementTemplatesFromFS()
        {
            //Get Object space 
            Bentley.CifNET.CadSystem.IObjectSpaceManager objectSpaceManager = Bentley.CifNET.ServiceManager.Instance.GetService<Bentley.CifNET.CadSystem.IObjectSpaceManager>();
            Bentley.CifNET.Objects.IObjectSpace objectSpace = objectSpaceManager.ObjectSpace;
            if (objectSpace == null) return;

            //Get or create ContentManagementModel
            Bentley.CifNET.ContentManagementModel.ContentManagementModel cmm = Bentley.CifNET.ContentManagementModel.ContentManagementModel.GetContentManagementModelAndCreateIfDontExist(objectSpace);
            if (cmm == null) return;

            //Get all feature symbology's 
            Bentley.CifNET.ContentManagementModel.GeometryAspectNameSet geometryAspectNameSet = cmm.GeometryAspectNameSet;

            Trace.WriteLine("Feature Symbology's having missing element templates: ");
            foreach (GeometryAspect geometryAspect in geometryAspectNameSet)
            {
                GeometricGeometryAspect geometricGeometryAspect = geometryAspect as GeometricGeometryAspect;
                if (geometricGeometryAspect == null) continue;

                //Get deafault template and 3D template for current feature symbology
                Bentley.CifNET.Model.ElementTemplate defaultElementTemplate = geometricGeometryAspect.DefaultElementTemplate;
                Bentley.CifNET.Model.ElementTemplate elementTemplate3D = geometricGeometryAspect.ThreeDElementTemplate;

                if (defaultElementTemplate == null || elementTemplate3D == null)
                {
                    //Report FS name having either default or 3D template missing
                    Trace.WriteLine(geometryAspect.Name.ToString());
                }
            }
        }

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Output