Hi. Welcome to KH. You could substitute any yarn that will work to gauge and that would mean a worsted weight should work. Beyond that, I assume it is quite similar to SS which means that if you don't sub with something similar your shawl will have a different drape. That might be OK. Maybe try a swatch with your chosen yarn and see if you like the way it works up in your pattern. HTH
This is the last time I'm frogging this project for the last time.
I haven't used the Eco. The Simply Soft is listed as a worsted weight #4. But my experience has been that it's more like a #3 sport weight. It's much thinner than Supersaver. When I've crocheted hats with both, it's been the same hook, same pattern but ends up with a one size smaller item with the Simply Soft.
Thank you so much for your answers, you are both right ,
I tried my yarn and I found it a little too stiff, so will look for a softer yarn.
Am pleased to be here and enjoy talking knit and crochet with you.