I went this route for a little while. I have a whole house softener using potassium salt with the addition carbon filter. I think it's safe to say that the residual potassium salt in the water is better than residual sodium for a planted aquarium. And I can't imagine the amount of salt a hobbiest typically might add is anywhere near what is left over after softening with sodium.
I had my softened water tested for potassium. It was 210ppm, so I decided to cut it with 50% RO. I was just being lazy about going 100% RO. But I didn't feel good about the leftover salts in the water. My TDS was 150-200 higher than my regular tap water. That increased TDS is from the residual salts in the water, either potassium or sodium. I just didn't feel comfortable with it after I started losing fish one by one over a six month period. I was also pushing CO2 at the time but thought the one by one deaths were odd.
So there is enough residual salts after softening to raise TDS substantially. Hence, there is something in the water, harmless or not, I dont know.
I've since gone to 100% RO and like knowing there's nothing extra in the water . I think softened water is an unknown.