I'm just wondering.. poisonous to who/what? Kids? Pets? How long are they poisonous for while decaying?

How long have people been 'treating and removing' them? Presumably the leaves were tolerable while on the tree, yet poisonous. It doesn't seem to stop people planting them. (A lot of questions, but there are a lot of experts on here!)

Normally to other plants, rarely for animals (see: poison ivy, poison oak and poison sumac). In really extreme cases can be very dangerous even to burn it (but nobody has a manchineel in their garden). Walnut can poison the soil for a couple of years so is better put its fallen leaves on a isolated compost pile. It depends on the species and the chemistry of the soil.

Wow, thank you, I had no idea. The walnut tree uses the poison to kill competitors, up to 20m away or more.

Black Walnut: The Killer Tree https://www.extension.iastate.edu/news/2005/jul/070701.htm

https://en.wikipedia.org/wiki/Juglone

"Juglone is occasionally used as a herbicide. Traditionally, ...has been used as a natural dye for clothing and fabrics...and as ink. ...has also found use as a coloring agent for foods and...hair dyes. ...is currently being studied for its anticancer properties"

And is not only the walnut, a lot of common trees do this in fact. Pines for example.

Except when the leaves form a coating over grass that grows mold, and you are allergic to mold, which will also kill the grass. You don't have to remove every last leaf, just enough to give the grass breathing room.

In fact I just spent six hours today doing that... with a rake. We had a window where the snow cover lifted just long enough. It'll rain tomorrow, then freeze and snow again later next week.