A letter in The Times this past week contains some inaccurate information. Reverse osmosis systems are extremely inefficient, using roughly three times the amount of water purified but do not use salt.
Conventional water softeners use salt in the process of clearing the filtering columns and do likely contribute to salt in the treated wastewater. Potassium can be used, but the cost is much higher and the residual potassium ions are probably just as harmful as salt.
It should be noted that if there were no water softeners in use at all, there likely would still be issues with growing grass in parks, etc. It was discovered that the reason for a major die-off of migratory waterfowl in the Imperial Valley in central California was due to “salts” leeched from the soil by the constant irrigation used in farming there. This result was confirmed by research conducted at Texas A&M, an agricultural school. See papers on using treated wastewater to grow cotton and others.
So, in a nutshell, eliminating water softeners will probably not solve the problem with growing green grass.