Here is my validation log that shows all the material expressions that are now missing for me if it helps.
Found disallowed object type ‘/Game/Athena/Environments/Island/Materials/Car_New.Car_New’
Found disallowed object type ‘/Script/Engine.MaterialExpressionBlackBody’
Found disallowed object type ‘/Script/Engine.MaterialExpressionTextureSampleParameterSubUV’
Found disallowed object type ‘/Script/Engine.MaterialExpressionInverseLinearInterpolate’
Found disallowed object type ‘/Engine/EngineSky/M_Sky_Panning_Clouds2.M_Sky_Panning_Clouds2’
Found disallowed object type ‘/Engine/EngineResources/Black.Black’
Found disallowed object type ‘/Engine/EngineMaterials/DefaultDiffuse.DefaultDiffuse’
Found disallowed object type ‘/Game/MigratedNew/BlackPlaceholder.BlackPlaceholder’
Found disallowed object type ‘/Game/MigratedNew/Placeholder_Normal.Placeholder_Normal’
Found disallowed object type ‘/Game/MigratedNew/WhitePlaceholder.WhitePlaceholder’
Found disallowed object type ‘/Script/Engine.MaterialExpressionPixelNormalWS’
Found disallowed object type ‘/Script/ControlRig.FKControlRig’
Found disallowed object type ‘/Script/NiagaraEditor.NiagaraMessageDataText’
Found disallowed object type ‘/Script/Engine.MaterialExpressionConstant4Vector’
Found disallowed object type ‘/Game/Creative/Devices/HologramDancer/LinearColorCurves/CurveAtlas_CP_Device_HologramDancer_01.CurveAtlas_CP_Device_HologramDancer_01’
Found disallowed object type ‘/Script/Engine.MaterialExpressionCollectionParameter’
Found disallowed object type ‘/Script/UnrealEd.FbxSkeletalMeshImportData’
Found disallowed object type ‘/Script/Engine.SkeletalMeshEditorData’
Found disallowed object type ‘/Game/Animation/Settings/FortniteAnimBoneCompressionSettings.FortniteAnimBoneCompressionSettings’
Found disallowed object type ‘/Engine/Animation/DefaultAnimCurveCompressionSettings.DefaultAnimCurveCompressionSettings’
Found disallowed object type ‘/Script/Engine.AnimDataModel’
Found disallowed object type ‘/Script/UnrealEd.FbxAnimSequenceImportData’
Found disallowed object type ‘/Script/Engine.MaterialExpressionParticleSubUV’
Found disallowed object type ‘/Script/Engine.MaterialExpressionBreakMaterialAttributes’
Found disallowed object type ‘/Script/Engine.MaterialExpressionIf’
Found disallowed object type ‘/Script/Landscape.MaterialExpressionLandscapeGrassOutput’
Found disallowed object type ‘/Script/Landscape.MaterialExpressionLandscapeLayerBlend’
Found disallowed object type ‘/Script/Landscape.MaterialExpressionLandscapeLayerSample’
Found disallowed object type ‘/Script/Landscape.MaterialExpressionLandscapeVisibilityMask’
Found disallowed object type ‘/Game/Environments/Apollo/Sets/Complex/Materials/MI_Order_Complex_Wall_01_3Layer.MI_Order_Complex_Wall_01_3Layer’
Found disallowed object type ‘/Script/FortniteGame.FortSkeletalMeshActor’
Found disallowed object type ‘/Game/Environments/Artemis/Sets/Blimp/Materials/MI_Blimp_Gondola_01.MI_Blimp_Gondola_01’
Found disallowed object type ‘/Game/Environments/Apollo/Sets/MountainBase/Props/Materials/MI_MountainBase_WallLight_01.MI_MountainBase_WallLight_01’
Found disallowed object type ‘/Game/Environments/FrontEnd/Materials/BP12/Emissive_Red_Midas_FE.Emissive_Red_Midas_FE’
Found disallowed object type ‘/Game/Environments/Apollo/Sets/Hightower_HC/Props/Materials/MI_HighTower_HC_BarrelCart_01.MI_HighTower_HC_BarrelCart_01’
Found disallowed object type ‘/Game/Environments/Artemis/Sets/Blimp/Materials/MI_Blimp_Gondola_Scaffolding.MI_Blimp_Gondola_Scaffolding’
Found disallowed object type ‘/Game/Gadgets/Zipline/Materials/MI_Zipline.MI_Zipline’
Found disallowed object type ‘/Game/Environments/Artemis/Sets/IOShared/Materials/M_IOShared_Logo.M_IOShared_Logo’
Found disallowed object type ‘/CRD_VFX_Spawner/Assets/Niagara/NS_Rain.NS_Rain’
Found disallowed object type ‘/Game/Creative/Sets/PropSets/Primitives/Rounds/Materials/MI_Primitive_Round.MI_Primitive_Round’
Found disallowed object type ‘/Game/Environments/Apollo/Sets/Hightower_HC/Props/Materials/MI_Hightower_HC_Chair_01.MI_Hightower_HC_Chair_01’
Found disallowed object type ‘/Game/Environments/Sets/Lab/Materials/MI_Lab_Fence_Dirty.MI_Lab_Fence_Dirty’
Found disallowed object type ‘/Game/Packages/Fortress_Floors_Generic/SM/Materials/MI_Fort_Floors_Generic_TilesClean.MI_Fort_Floors_Generic_TilesClean’
Found disallowed object type ‘/Game/Playgrounds/Items/Cubes/TextureData/Cube_Black.Cube_Black’
Found disallowed object type ‘/Game/Playgrounds/Items/Cubes/Materials/M_C_Cube_Black.M_C_Cube_Black’
Found disallowed object type ‘/Game/Environments/Apollo/Sets/MountainBase/Props/Meshes/S_MountainBase_WallLight_01.S_MountainBase_WallLight_01’
Found disallowed object type ‘/Script/Landscape.LandscapeLODStreamingProxy’
Found disallowed object type ‘/Game/Environments/Sets/Cabin01/materials/M_FORT_Logcabin_Walls.M_FORT_Logcabin_Walls’
Found disallowed object type ‘/Game/Environments/Artemis/Sets/IOShared/Materials/MI_IOShared_Antenna_Flash_A.MI_IOShared_Antenna_Flash_A’
Found disallowed object type ‘/Game/Athena/Environments/Island/Materials/MI_pickup_truck_rusted_02.MI_pickup_truck_rusted_02’
Found disallowed object type ‘/Game/Building/ActorBlueprints/Prop/Car_DEFAULT.Car_DEFAULT_C’
Found disallowed object type ‘/Game/Athena/Environments/Island/Materials/PickupIsland.PickupIsland’
Found disallowed object type ‘/Game/Athena/Apollo/Environments/Landscape/Materials/NonRVT/M_Apollo_Terrain_Basic_Inst.M_Apollo_Terrain_Basic_Inst’
Found disallowed object type ‘/Game/Athena/Items/Consumables/ZipLine/SplineZipline/SM_ZiplineCylinder.SM_ZiplineCylinder’
Found disallowed object type ‘/Game/Athena/Items/Consumables/ZipLine/SplineZipline/Material/M_ZiplIne_SplineThicken.M_ZiplIne_SplineThicken’
Found disallowed object type ‘/Game/Athena/Items/Consumables/ZipLine/SplineZipline/Material/M_ZiplIne.M_ZiplIne’
Found disallowed object type ‘/Game/Environments/Sets/RaceTrack/Materials/M_Racetrack_Flag.M_Racetrack_Flag’
Found disallowed object type ‘/Game/Environments/Artemis/Sets/IOShared/Materials/MI_IOShared_Railings_2layer.MI_IOShared_Railings_2layer’
Found disallowed object type ‘/Game/Environments/Apollo/Props/Flagpole/Materials/MI_Flagpole_Cam_Cod.MI_Flagpole_Cam_Cod’
Found disallowed object type ‘/Game/Environments/Apollo/Sets/Tinfoil/Materials/MI_Tinfoil_CockpitScreen_A_A.MI_Tinfoil_CockpitScreen_A_A’
Found disallowed object type ‘/Game/Creative/Environments/Meshes/Meteor/CP_Meteor_Small_Fire_Mesh_Particles.CP_Meteor_Small_Fire_Mesh_Particles’
Found disallowed object type ‘/Game/Athena/Effects/Materials/Meteor/M_Meteor_Small_Fire_NoTick.M_Meteor_Small_Fire_NoTick’
Found disallowed object type ‘/Game/Athena/Environments/Island/Materials/MI_kcar2_Island.MI_kcar2_Island’
Found disallowed object type ‘/Game/Environments/Sets/Lab/Materials/Lab_Stadium_Floodlight.Lab_Stadium_Floodlight’