We've been using pallets to build a bunch of raised beds in our
yard, so I've done a lot of research on the treated/ untreated issue. What I've found, is that most pallets have stamps that say how they have been treated. HT= Heat-Treated, DB = De-Barked, KD = Kiln-Dried. I've found all of those codes on different pallets I've gotten, and they are all fine.
There are a few others that I've heard of, but have not seen in person: CT = Chemically Treated, MB = Methyl Bromide, PT = Pressure Treated. All of those involve nasty chemicals, according to my research, but I haven't actually seen those ones yet in person. Sometimes there are also stamps that tell what the pallet was carrying, so every time I see a new code, I Google it, and can usually figure out what it means.
My understanding is that this chemical treatment is only required for pallets used for international shipping.
That pallet garden looks cool, btw. I'd love to see how it works out.