Doubt many here would be using various powerful acids found in laundry sours at nearly levels to harm septic system. That is unless doing huge amounts of laundry per day, and copious amounts of chemicals. Still if concerned I'd run things by clean out service guy to be sure.
https://www.paradisevalleyseptic.com/keep-your-septic-system-healthy-by-avoiding-the-dirty-dozen/
There are all sorts of laundry sours for commercial/industrial use; some are also softeners, others aren't.
Personally would stick to distilled white vinegar for domestic use for host of reasons.
https://en.wikipedia.org/wiki/Laundry_sour#:~:text=A laundry sour is a,of detergents and rust stains.
Many of these laundry sours are potent and concentrated. They are designed like most commercial/industrial laundry products for dosing in machines starting at 50lbs upwards to over 200lbs. Unless dosage information is given with product, and or even if it is, you're going to have to do sums to scale dosage down to a washer that holds only 11 lbs to 18 lbs.
Also keep in mind industrial/commercial washing machines are designed and built to withstand repeated use of commonly used harsh products. Unlike domestic washers most industrial don't have pumps; thus final rinse water that remains highly or even moderately acidic (from using too much sour) sitting for several hours or days isn't going to harm anything.
OTOH many commercial/industrial products state clearly on package "not for use in homestyle washing machines" or words to that effect for a reason.
Then you have fact cellulose fibers such as cotton and linen are harmed by exposure to strong or even moderate acids. The latter is more susceptible, but both can be damaged in long run with repeated use, or just one rinse where pH is too high.
Ideally sour dosage will correspond to remaining alkaline pH level of textiles by final rinse so the two will cancel each other out. This results in a neutral pH by end of cycle.
In theory rinsing thoroughly and long enough will bring down pH on its own since water is normally in neutral pH range.
If moderate to near neutral products were used in laundering, souring as part of rinsing isn't always required. This is one reason why you've seen commercial laundries move away from brute force of strong chemicals, to more milder liquid products where possible. Not having to use a sour rinse saves not just on chemical costs, but water, time, etc.....
