But I don't see in my searches where it mentions that mammals don't have this mechanism anymore? You'd think the Wikipedia would mention it.
Cave fish that haven't been isolated as long (more then 3 million years) still have it. So it's really really weird that placental mammals (which includes all non egg laying or marsupial mammals, including us) don't have it. It lends additional support to the Nocturnal Bottleneck theory https://en.wikipedia.org/wiki/Nocturnal_bottleneck
UV rays do not enter well in the deep water layers so marine fishes living under 60m deep shouldn't have any use for this mechanism. Why mammals had lost it and those fishes still keep it?
I would suggest to read the former statement as "but most other creatures are still untested for the mechanism".
Or maybe we are missing other possible explanations here. I'll add a different hypothesis: "because (or this is evidence that?) placentate mammals have evolved in the dark of a womb and other animals have external embryos (like kangaroos) or have delicate eggs".
My hypothesis would be that the real purpose of this mechanism is protecting the embryo development when even a small punctual UV damage could trigger a cascade of much more serious consequences. This would explain why marsupials and other fishes still need it but diurnal placentates not.
Crocodyles, Platypuses or marine turtles that dig nests in the soil still keep this mechanism?.