Article from 2022 reviewing data from Illinois 1981-2017 and primarily addressing crippling rates, which did increase after the lead ban but dropped to the pre-ban levels, likely related to limited the distance of shots taken
Fewer duck hunters, and more geese hunters
https://onlinelibrary.wiley.com/doi/full/10.1002/wlb3.01001Black duck lead levels in N.J. 1978 to 2017 from the
Journal of Fish and Wildfowl Management, 6-2021
"The prevalence of ducks with blood lead levels 1.0 ppm, considered clinically evident toxicity, declined from 19% in 1978 to 1% in 2017."
The abstract however does not estimate a change in black duck mortality.
This is the only article I could find estimating the decrease mortality after the ban; in the July 2000
Journal of Wildlife ManagementThe article was cited by Brent back in 2006
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=243004&page=6The thread went to 26 pages
We examined the extent to which ingested nontoxic (steel and bismuth-tin) shotgun pellets replaced toxic (lead) pellets in ducks harvested in the Mississippi Flyway during the 1996 and 1997 hunting seasons (fifth and sixth yr after nationwide conversion to nontoxic shot). Gizzards were collected from 16,651 ducks and processed for the presence of pellets. Prevalences of ingested pellets were 8.9% for 15,147 mallards (Anas platyrhynchos), 12.7% for 749 ring-necked ducks (Aythya collaris), 4.3% for 579 scaups (Aythya affinis and A. marila), and 9.7% for 176 canvasbacks (Aythya valisineria). For gizzards with ingested pellets, as much as 68% of mallard, 45% of ring-necked duck, 44% of scaup, and 71% of canvasback contained only nontoxic pellets.
We estimated that nontoxic shot reduced mortality from lead poisoning in Mississippi Flyway mallards by 64%. Ingestion of ≥2 toxic pellets declined by as much as 78%. To the extent that our findings apply to other species and flyways in North America, an estimated 1.4 million ducks in the 1997 fall continental flight of 90 million were spared from fatal lead poisoning.
That "64%" reduction is repeated now over and over in every Green blog and website
The survival data is complicated by the fact that there is only a 30-50% first years survival rate for ducks, and deaths from avian influenza A(H5N1), avian cholera & botulism, and aflatoxicosis have increased
