"Uncheck all" in Gizmos menu in Scene view?

There is no official way (none I am aware of), but depending on your willingness to dig into deep private reflection stuff of UnityEditor.dll, you could write some editor code to do this.

Basically from what I see from the decompile, class AnnotationWindow controls the UI. It uses AnnotationUtility.GetAnnotations to obtain information about what classes have gizmos available and uses AnnotationUtility.SetGizmoEnabled to enable/disable the gizmo (SetIconEnabled to enable/disable the icon).

Let me try... yea, this one should work.. here you go: Menu option "Window / Disable All Gizmos" to turn off everything in a blink.

 using UnityEditor;
 using System;
 using System.Reflection;
 
 public class DisableAllGizmos
 {
     [MenuItem("Window/Disable All Gizmos")]
     static void DisableAllGizmosMenu()
     {
         var Annotation = Type.GetType("UnityEditor.Annotation, UnityEditor");
         var ClassId = Annotation.GetField("classID");
         var ScriptClass = Annotation.GetField("scriptClass");
         
         Type AnnotationUtility = Type.GetType("UnityEditor.AnnotationUtility, UnityEditor");
         var GetAnnotations = AnnotationUtility.GetMethod("GetAnnotations", BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static);
         var SetGizmoEnabled = AnnotationUtility.GetMethod("SetGizmoEnabled", BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static);
         var SetIconEnabled = AnnotationUtility.GetMethod("SetIconEnabled", BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static);
 
         Array annotations = (Array)GetAnnotations.Invoke(null, null);
         foreach (var a in annotations)
         {
             int classId = (int)ClassId.GetValue(a);
             string scriptClass = (string)ScriptClass.GetValue(a);
 
             SetGizmoEnabled.Invoke(null, new object[] { classId, scriptClass, 0 });
             SetIconEnabled.Invoke(null, new object[] { classId, scriptClass, 0 });
         }
     }
 }