I could not say if it is toxic. I will say that I have done it and (anecdotally) saw no ill effects on neither plants nor animals.

It being old is probably good since it may have leached out anything bad over time.

I've even used that cheap plastic sheeting from Lowes. To me one of the biggest appeals of Bog gardening is that it is vastly less expensive than water gardening. I've made really fantastic bog gardens with less that $20.00 in materials.