The myth about getting minerals from water is just that. When I went RO I really researched it. Seems most of the misinformation generated online is from the bottled water/"drinking water" industry to keep you from going RO. You get your minerals from the plants you eat...The plants that grow in the soil that contains the minerals and elements your body needs. There are people in other parts of the world who only get their drinking water from rain collection. In this country, for many years it is how we used to get our drinking water. Not everyone had a shallow well or spring. And we didn't have electric pumps to bring it from the ground. Ever hear of a cistern? Every older house had one that was fed by the gutters and downspouts from the house.
And yes, softened water and its sodium content killing plants is another myth. Now the water discharge water during the regen is a different story. It could have up to 3 lbs of salt per gallon and will kill plants. And no regen wastewater will not harm your septic. It gets diluted with the other water and the bacteria will grow to be tolerant of the salt, just like freshwater, brackish, or saltwater aquariums.
There is roughly 560 milligrams of salt in a gallon of softened water. That's about 1/5 of a teaspoon. The real reason we don't soften spigot water isn't because of the plants, it's because of the volume. If you watered your lawn you could have to regen daily...Or worse, run out of softened water and even potentially foul the resin bed. Some softeners have clocks instead of meters and will only regen after a specific time.