Favorite way to create an new IEnumerable<T> sequence from a single value?

Your example is not an empty sequence, it's a sequence with one element. To create an empty sequence of strings you can do

var sequence = Enumerable.Empty<string>();

EDIT OP clarified they were looking to create a single value. In that case

var sequence = Enumerable.Repeat("abc",1);

I like what you suggest, but with the array type omitted:

var sequence = new[] { "abc" };

Or even shorter,

string[] single = { "abc" };

I would make an extension method:

public static T[] Yield<T>(this T item)
{
    T[] single = { item };
    return single;
}

Or even better and shorter, just

public static IEnumerable<T> Yield<T>(this T item)
{
    yield return item;
}

Perhaps this is exactly what Enumerable.Repeat is doing under the hood.

or just create a method

public static IEnumerable<T> CreateEnumerable<T>(params T[] items)
{
    if(items == null)
        yield break;

    foreach (T mitem in items)
        yield return mitem;
}

or

public static IEnumerable<T> CreateEnumerable<T>(params T[] items)
{
   return items ?? Enumerable.Empty<T>();
}

usage :

IEnumerable<string> items = CreateEnumerable("single");