It depends on your climate (where be you?) but in many locations bed rot is pretty trivial. Some places it's wild! I've had boards in the ground for four years (when I redid everything) that show NO sign of rot. They're plain old ordinary untreated Douglas Fir dimensional lumber held together with plain old nails.

Polyethylene will break down in the soil but It's glacially slow. Toxic plasticizers like the recently damned pthalates are not usually added to polyethylene. It's about as non-toxic a plastic as you'll find. YMMV

Keep in mind that much cheaper, and thinner, may also mean a much shorter life for that polyethylene liner.

It would not be a problem with toxic substances leaching into the soil, but since it is moisture, not soil, that causes wood to rot it will do little to prevent that.

kimmq is kimmsr

sure. Poly has very low toxicity as it is just synthesized out of ethylene stock. It will last a while. I lined my basement on the outside, and I can tell it is still there 6 years later.