Randy Holmes Farley has tested this and says it is a very negligible amount for most foods and not worth the effort. You also lose certain things like potassium but that isn't necessarily a problem depending on the tank. I could see how PE mysis might be different because there is a ton of small particulate but with typical mysis the water is clear.I seriously lowered my phosphates by floating my frozen cubes in display water and then draining all the liquid. I know people do that for PE Musis. But oretty much all frozen foods have phosphate that can easily be removed
edit:
https://reefs.com/magazine/aquarium-chemistry-phosphate-and-math-yes-you-need-to-understand-both/