If a bar of soap lasts, say, 2-4 weeks, and each bar weighs about 5 ounces, and half of the bar is salt by weight, then that means you are putting 2.5 to 5 ounces of salt into your septic system every 4 weeks.
If you have a home water softener, you might use 50 pounds (800 oz) of salt every 8 weeks or so -- possibly more depending on hardness. That's about 400 oz of salt going into your septic system every 4 weeks.
I don't worry about using my home water softener. So I wouldn't worry about using salt bars with my septic system.
The concern about salt in the soil is more about whether or not the salt is well diluted and whether or not the salt is washed away by rainfall. Salt if used to kill weeds is a concentrated application to foliage and the surface soil where roots grow. Salt in a septic system is highly diluted and is put into the below-ground soil. In an arid climate (one with little rainfall), accumulation of salt can cause problems over time. In a climate with a lot of rainfall, it's pretty much not an issue.