VO.cs

public abstract class Comment 
{
    public static ProposedComment Parse(String proposedComment)
    {
        return new ProposedComment(proposedComment);
    }
}

public class ProposedComment : Comment 
{
   public ProposedComment(String value)
   {
       Value = value;
   }
   
   public Value { get; }
}

public abstract class ReviewedComment : Comment
{
    ...
}

public class CleanComment : ReviewedComment
{
    ...
}
    
public class ProfaneComment : ReviewedComment
{
    ...
}

public interface ReviewComment
{
    ReviewedComment Review(ProposedComment comment): ReviewedComment;
}

public class Program
{
    ReviewComment rc;
    
    public static void Main(String[] args)
    {
        // As a caller, all I need to know is the Comment.Parse method. The type system guides me from here
        ProposedComment p = Comment.Parse("comment");
        ReviewedComment r = rc.Review(p);
        
        // Make all states explicit and handle them at compile time. Avoid throwing hidden exceptions at runtime.
        switch (r) 
        {
            case CleanComment cl:
               ...
            case ProfaneComment pr:
               ...
        }
    }
}

I'd go for var p = ProposedComment.Parse("comment"); putting the Parse on ProposedComment. Because that's what you're starting out with. Why do ProposedComment and ReviewedComment share a base type? I'd totally decouple unless there's some behavior that needs both.