They used to suggest sewing wool with silk, silk with silk, and cotton with cotton, and it's still works well as a rule. These days polyester and rayon (mostly embroidery as far as I know) have taken over nearly every use of silk, and I can't say it does a bad job. I have hardly ever had trouble with threads cutting the fabric. The exception has been a few cases of terrible lycra-cotton-viscose jersey.
Spools of slik threads are still in the shops, but they hardly ever suggest using it for regular purposes. Like with everything it has a lot to do with price and there's never enough silk made to replace synthetic fibers. Silk is the strongest thread, but it's only breakage tests it comes out favorably. Silk will weaken when exposed to sun, long term humidity, most types detergents. It's not practical for all types of project, but it might still be the best for some.