Baking Soda is not harmful to septic systems but instead baking soda and vinegar can actually help to maintain your septic system which helps you avoid clogs and other septic system emergencies.
You can use baking soda and vinegar as a natural cleaning agent for toilets, sinks and other drains which will not only clean your sinks and toilets but when the baking soda and vinegar are washed down the drain and reaches your septic system it will help with the bacteria that's needed in the septic tank and keep the septic system working properly.
To make your own homemade cleaning agents you can mix a 1/4 cup of baking soda with 1/2 cup of vinegar and 2 tablespoons lemon which helps the septic system plus does a great job of cleaning.