This practice is widely known and derided enough to have a nickname: “dead dunking.” My paternal seventh great grandfather has been dead dunked multiple times judging by erroneous records in the LDS database, they have hime all over the place, even in NC in 1660 (he wasn’t, he was barely off the boat in St. Mary’s City, MD). I know why, a branch of the family converted and went west prior to the Civil War. He was a devout man, professed himself pardoned for his sins and forgiven by the Lord in his last will and testament. Sort of insulting actually. I can see why Holocaust victims are upset, it implies that they were deficient and hell-bound without Mormon intervention, as if Mormons are some bastion of sinlessness.
This is the reason they’re so big on genealogy. Taking names for mass Baptism of the dead.
https://en.wikipedia.org/wiki/Baptism_for_the_dead#Genealogy_and_baptism