RemoveByKey.cs

private static IEnumerable<KeyValuePair<TKey,TValue>> RemoveByKey<TKey,TValue>(
    this IEnumerable<KeyValuePair<TKey,TValue>> pairs,
    string key) 
{
    var index = pairs.Select(x => x.Key).ToList().FindIndex(x => x.Equals(key));
    
    if (index >= 0)
    {
        var result = pairs.ToList();
        result.RemoveAt(index);
        return result;
    }
    
    return pairs;
}