Some solar panels are considered hazardous waste, and some are not
That's horribly unspecific
Lead and cadmium
AFAIK in EU Leaded solder is forbidden since many years (outside of safety related equipment). Cadmium is regulated in similar fashion. Electronic equipment disposal and recycling is also strictly controlled.
The whole thing looks like fear mongering.
Nobody talks about dumping these things in the field. Glass, silicon, and aluminum can be reused or disposed safely. As I said in previous post - in EU electronic waste is disposed properly without dumping whole things into a landfill. Solar panels aren't even the hardest e-waste to deal with.