Adding a few comments.

Rx.NET/Source/src/System.Reactive/Linq/Observable/DistinctUntilChanged.cs

@@ -26,8 +26,10 @@ namespace System.Reactive.Linq.ObservableImpl
             return _source.SubscribeSafe(sink);
         }
 
-        class _ : Sink<TSource>, IObserver<TSource>
+        private sealed class _ : Sink<TSource>, IObserver<TSource>
         {
+            // CONSIDER: This sink has a parent reference that can be considered for removal.
+
             private readonly DistinctUntilChanged<TSource, TKey> _parent;
             private TKey _currentKey;
             private bool _hasCurrentKey;

Rx.NET/Source/src/System.Reactive/Linq/Observable/Do.cs

@@ -168,6 +168,8 @@ namespace System.Reactive.Linq.ObservableImpl
 
             private sealed class _ : Sink<TSource>, IObserver<TSource>
             {
+                // CONSIDER: This sink has a parent reference that can be considered for removal.
+
                 private readonly Actions _parent;
 
                 public _(Actions parent, IObserver<TSource> observer, IDisposable cancel)