Even vintage detergents often contained both washing soda and STPP. The benefit of the washing soda is that it increases the pH more than STPP by itself. The washing soda became important as a way to hold the phosphate level to no more than 8% elemental phosphorus by weight. Older formulations often also contained copious amounts of sodium sulfate, which has little to no function other than to bulk up the product and perhaps make it pour easier. "Ultra" formulations simply eliminate the sodium sulfate, with little to no functional impact, other than requiring ounces rather than cups for proper wash load dosing.
For HE or front loader machines, a potential advantage of "cutting" the detergent with STPP is that it can lower the amount of surfactant, which in turn lowers the sudsing, while maintaining maximum build/break/water softening capability. If one is concerned about the suds level being too low, one can always add more of the mix, say, 1/3 more than one usually might add. That way the proportions would be no different than if one were to add, say, 1 oz of original detergent and 1/2 oz of STPP, by weight. Do you have a suitable scale in your laundry room for that?
None of this, however, is all that critical. If one adds 20% or 40% STPP I doubt it will make much difference in the wash results. The important thing is to add enough of the total mix to loosen and suspend all the dirt and allow it to be rinsed away.
