Are there any sound designers who only work at composing/producing sound for products such as digital applications, wearables, home electronics, appliances, etc., or is it too small of a market for specialization?
Henry Daw is ex-Nokia, ex-Microsoft and has set up to do exactly that. His studio is called Oblique Sound http://www.obliquesound.com/
Just plugging a mate :)
Boom libraries do one. I've not heard it but all their libraries are great.
I'm sure this cool dj did a lot of SFX for apps and phones back in the day.
Another certain truth is that sound design is a broad discipline and specializing in one field for marketing purposes will only have you lacking in other directions.