Soap
I use bar soap as a pre-treatment for stains and it works even on stains where it is not supposed to work on - maybe because it loosens the dirt better, so it can be attacked better by the detergent in the wash-water.
Also I mix my own stain pre-treatment soap according to an old recipe I found in an old book.
There are some things to keep in mind when using soap!
Hard water is poison to soap! Even when rinsing in hard water areas there must be a softener (calgon, phosphate, soda or anything like that) added to the first warm or better hot rinse!
As already said: warm (40°C) is the minimum wash-temperature, hot is better and boil-wash (100°C) is always best with soap!
Soap does not bleach any stains and so needs the use of oxygene or chlorine bleach and does give a yellow-grey appearance after a while when not used together with a brightener or blueing!
But the appearance of the washing after being treated with soap is better than with any detergent. The smell is not artificial like that of detergents. Ironing is done much faster and fabrics smooth out much better when line dried than those washed with detergent. Also the water absorbance of towels is better. And no softener is to be needed!
Here in Germany are soap-based detergents available - see link below - but they are more expensive and are not made to wash with cold water which I generally do. But with warm or hot water they can be used like all other detergents, too!
Ralf