Shouldn't ILookup<TKey, TElement> be (declared) covariant in TElement?

Question

The definition System.Linq.ILookUp<TKey, TElement> reads

interface ILookup<TKey, TElement> : IEnumerable<IGrouping<TKey, TElement>>, IEnumerable 
{
    int Count { get; }

    IEnumerable<TElement> this[TKey key] { get; }

    bool Contains(TKey key);
}

Since IEnumerable is covariant in IGrouping<TKey, TElement> , IGrouping<TKey, TElement> is covariant in TElement and the interface only exposes TElement as a return type, I would assume that ILookup is also covariant in TElement. Indeed, the definition

interface IMyLookup<TKey, out TElement> : IEnumerable<IGrouping<TKey, TElement>>, IEnumerable 
{
    int Count { get; }

    IEnumerable<TElement> this[TKey key] { get; }

    bool Contains(TKey key);
}

compiles without problems.

So, what might be the reason why the out keyword is missing in the original definition? Might it be added future versions of Linq?