Some interesting speculation here - and that's all! Amphibians don't eat the seeds so we needn't worry about them. Of detritivorous invertebrates, few of them will eat freshly fallen leaves or seeds. There's certainly no suggestion that the toxins contaminate ground water.
The number of 'poisonings' per year is a matter of interpretation and definition: many of these will be children who are taken to hospital for treatment even though no symptoms occur; most poisonings are mild (note that one of the first symptons is
vomiting - it's pretty much a self-restricting process). I can't find any data for number of deaths from laburnum-poisoning - it would need to be someone taking a pretty big dose: most likely wilfully.
We shouldn't forget that many (possibly most) leguminous plants are 'poisonous' in part to some extent. See what happens if you eat uncooked red kidney beans, for instance .......
PS: When I was a lad I my infant and junior schools were on a street called Laburnum Crescent which, as you might guess, was lined with large, prolific laburnum trees. Everyone was warned not to eat the pods (not that anyone ever had any inclination to do so) and during my years living it that area (around twenty) no one ever suffered poisoning from them. Yet every year there was a parent or two saying that the trees should be chopped down because of their threat to human life ..... I suspect that they wanted the space to park their cars on ......
