SkipLast.cs

// Based on https://codereview.stackexchange.com/a/223655/145828
static public IEnumerable<T> SkipLast<T>(this IEnumerable<T> data, int count)
{
  if (data == null) throw new ArgumentNullException(nameof(data));
  if (count <= 0) return data;

  if (data is ICollection<T> collection)
    return collection.Take(collection.Count - count);

  IEnumerable<T> Skipper()
  {
    using (var enumer = data.GetEnumerator())
    {
      T[] queue = new T[count];
      int index = 0;

      while (index < count && enumer.MoveNext())
        queue[index++] = enumer.Current;

      index = -1;
      while (enumer.MoveNext())
      {
        index = (index + 1) % count;
        yield return queue[index];
        queue[index] = enumer.Current;
      }
    }
  }

  return Skipper();
}