Steps to reproduce:
1. Create new Unity project using Universal 3D Template
2. In Hierarchy create Button object (Right-click > UI Toolkit > Standard Elements > Button)
3. In “Open UI stage” pop-up press “Continue” and save UXML file
4. While Button element is selected, in Inspector expand “Backdrop Filter”
5. Select Custom function

Actual results: The Backdrop Filter section in the UI Authoring Inspector exposes "Custom Function" as a selectable option. Selecting it applies the unsupported filter and throws "Custom filters are not supported for backdrop-filter on element ''. Custom filters will be ignored." warning in the Console window

Expected results: The "Custom Function" option is not available (or is disabled) in the Backdrop Filter section, consistent with UI Builder behavior, no warnings are thrown in the Console

Reproducible with versions: 6000.6.0b1, 6000.6.0b6, 6000.7.0a3

Can’t test with versions: 6000.0.80f1, 6000.3.21f1, 6000.5.6f1, 6000.6.0a7 (UI Authoring not available)

Tested on (OS): macOS Tahoe 26.5.2 (M1), Ubuntu 24.04

Custom filters are not supported for backdrop-filter on element ''. Custom filters will be ignored.
UnityEngine.UIElements.BaseRuntimePanel:Render ()
Unity.UIToolkit.Editor.PanelElement/RuntimePanel:Render () (at /home/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Controls/PanelElement/PanelElement+Panel.cs:48)
Unity.UIToolkit.Editor.PanelElement:FrameUpdate () (at /home/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Controls/PanelElement/PanelElement+Rendering.cs:171)
Unity.UIToolkit.Editor.VisualElementEditingStage:ProcessDelayedCommands () (at /home/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Stages/VisualElementEditingStage.cs:741)
Unity.UIToolkit.Editor.VisualElementEditingStage:OnStylingChanged (Unity.UIToolkit.Editor.CommandContext&) (at /home/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Stages/VisualElementEditingStage.cs:723)
Unity.UIToolkit.Editor.CommandSystem:InvokeHandlers (Unity.UIToolkit.Editor.Command,Unity.UIToolkit.Editor.CommandContext) (at /home/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Commands/CommandSystem.cs:334)
Unity.UIToolkit.Editor.CommandSystem:ExecuteCommandAndInvokeHandlers (Unity.UIToolkit.Editor.Command) (at /home/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Commands/CommandSystem.cs:297)
Unity.UIToolkit.Editor.CommandSystem:Execute (Unity.UIToolkit.Editor.Command) (at /home/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Commands/CommandSystem.cs:186)
Unity.UIToolkit.Editor.UICommandQueue:Execute (Unity.UIToolkit.Editor.Command) (at /home/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Commands/UICommandQueue.cs:65)
Unity.UIToolkit.Editor.SetInlineStylePropertyCommand`1<UnityEngine.UIElements.StyleList`1<UnityEngine.UIElements.FilterFunction>>:Execute (object,UnityEngine.UIElements.VisualElement,UnityEngine.UIElements.StyleSheets.StylePropertyId,System.Action`3<UnityEngine.UIElements.StyleProperty, UnityEngine.UIElements.StyleSheet, UnityEngine.UIElements.StyleList`1<UnityEngine.UIElements.FilterFunction>>,UnityEngine.UIElements.StyleList`1<UnityEngine.UIElements.FilterFunction>) (at /home/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Commands/SetInlineStylePropertyCommand.cs:38)
Unity.UIToolkit.Editor.StylePropertyBinding:ProcessChange<UnityEngine.UIElements.StyleList`1<UnityEngine.UIElements.FilterFunction>> (UnityEngine.UIElements.StyleList`1<UnityEngine.UIElements.FilterFunction>,Unity.UIToolkit.Editor.StyleInspectorElement/AuthoringContext,Unity.UIToolkit.Editor.StylePropertyBinding,System.Action`3<UnityEngine.UIElements.StyleProperty, UnityEngine.UIElements.StyleSheet, UnityEngine.UIElements.StyleList`1<UnityEngine.UIElements.FilterFunction>>) (at /home/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Inspector/Bindings/StylePropertyBinding.cs:335)
Unity.UIToolkit.Editor.StylePropertyBinding:ProcessChange<UnityEngine.UIElements.StyleList`1<UnityEngine.UIElements.FilterFunction>> (UnityEngine.UIElements.ChangeEvent`1<UnityEngine.UIElements.StyleList`1<UnityEngine.UIElements.FilterFunction>>,Unity.UIToolkit.Editor.StylePropertyBinding/CallbackContext,System.Action`3<UnityEngine.UIElements.StyleProperty, UnityEngine.UIElements.StyleSheet, UnityEngine.UIElements.StyleList`1<UnityEngine.UIElements.FilterFunction>>) (at /home/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Inspector/Bindings/StylePropertyBinding.cs:435)
Unity.UIToolkit.Editor.StylePropertyBinding:ProcessChange (UnityEngine.UIElements.ChangeEvent`1<UnityEngine.UIElements.StyleList`1<UnityEngine.UIElements.FilterFunction>>,Unity.UIToolkit.Editor.StylePropertyBinding/CallbackContext) (at /home/bokken/build/output/unity/unity/Modules/UIToolkitAuthoringEditor/Managed/Inspector/Bindings/Generated/StylePropertyBinding.gen.cs:991)
UnityEditor.GUIView:ProcessEvent (UnityEngine.EntityId,intptr,bool&) (at /home/bokken/build/output/unity/unity/Editor/Mono/GUIView.cs:82)