Don't get me wrong, a few people using it won't change a lot.
But overdosing it by much, somehow getting it into any kind of open water (rivers, lakes, oceanes) can indeed damage the enviroment including animal stock as well as plant stock.
Nonsense. STPP has very low toxicity to animals. In fact, it's approved for use as a humectant in various human foods. For plants, it would be less toxic than table salt.
The issue you may be thinking of is eutrophication. Which basically means an overgrowth of algae in still water bodies. That would exclude streams, rivers, and the ocean. And eutrophication is not triggered solely by phosphates. It also requires a surplus of nitrates, which are a main component of sewage. If sewage effluent were adequately treated to reduce nitrate load, then phosphates wouldn't be an issue. They are also not an issue in many of the more arid parts of the United States, where nitrate pollution is the main concern.
Additionally, the healthy human body requires phosphates for survival, and excretes as much phosphate into the sewage system as a home washer would using phophated detergent. So one may want to consider bagging one's own poop and burying it in the back yard, too.
If you have soft water, you probably won't need it at all.
Also not true. Water softening is only part of STPP's value. There are hard water minerals in most dirt and soil, and these create precipitates with other water softeners like sodium carbonate, which redeposit on the fabrics. Also, the ability of complex phosphates (of which STPP is one) is that they literally grab the dirt off the fabric and keep it in solution, making a phosphated detergent far more effective than a non-phosphated formula.
If you are on a proper sewer system with a state-of-the-art water purification system, your impact will be minimal.
It is true that advanced municipal tertiary sewage treatment systems can remove excess phosphate from the sewage stream. This should be done anyway due to aforementioned phosphate load from normal human excretion.
Funny side note: We here in the EU mostly replaced phosphates in laundry detergents with something called Zeolith A.
It basicly is a 3D-nanostructre, accomplishing simmilar effects as phosphates in term of water softening. Meanwhile, its not toxic (STPP can have harmfull effects, though only if ingested in high quantities), does not get into the biological system of any species and as far as I know is mostly removed even by low standard water treatment.
Zeolites are less effective than phosphates at water softening. Additionally, they don't have the break/cleaning ability of phosphates. Zeolites are aluminosilicates, a form of clay with very small particles and microscopic pores, and if ingested in high quantities can certainly be toxic, while STPP is an approved food additive. In fact, the clay mud that zeolites produce in the waste stream is said to be an increasing problem for European sewage treatment facilities.
Typically I recommend boosting a regular powdered laundry detergent about 33% by weight with STPP. This gives about an 8% phosphorus content, similar to what phosphated laundry and dishwashing detergents once had.