I had a similar question and found both your post and this scientific paper
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC167528/pdf/612566.pdf
The PhD researchers from Michigan State examined the fate of 2,4-D in compost. After 50 days under what sound like ideal conditions (fine chopping, appropriate temperature and moisture) for breakdown of organic matter, composting was essentially complete. They found that about half the 2,4-D was converted to carbon dioxide after 50 days, i.e. was completely broken down (a quarter of it was gone within the first 10 days). After 50 days, the remainder of the original 2,4-D was 1) in very large molecules, i.e., had been broken apart and made into other compounds, or 2) was very tightly adsorbed onto organic matter.
Interpretation:
After composting to completion (which will take longer than the 50 days this study), there are only tiny amounts of active 2,4-D in the compost. After addition to the soil, any herbicide in the soil will presumably continue to degrade. Although plants roots can take up the salt form of 2,4-D, a miniscule proportion of thein original 2,4,-D will get into the plants and even less into the parts we eat. Given that mammal kidneys very readily excrete 2,4-D, there seems to be extremely low risk of moderate amounts of 2,4-D in material that are well-composted.
No, there is not absolutely zero 2,4-D, _but remember_ that no field or garden, even hard-core organic, is completely free of pesticides (unfortunately!) and also that many natural plant compounds are potentially toxic to humans. This risk seems on par with those very, very low risks (no, absolutely pure does not exist; don't bother trying!). So as a scientist and organic (but pragmatic) gardener, to be conservative I will avoid composting grass recently treated with 2,4-D but will not categorically exclude grass that was treated some time ago as long as it does not make up a substantial proportion of my compost material.
