I believe that any arsenic that was used on the wood would have long since been washed away in the soil. The copper content, if it was still harmful, I think would kill any plant near it well before it was anything like ready to eat.
If it was possible to remove the remains of this wood I probably would, but I think there would be little risk after all this time.
In a new garden, a soil test is a good idea anyway, and I understand it can be accomplished without too much expense. This may put your mind at rest. Better safe than sorry.
You say the remains of this wood are near your new garden, but not in it. Probably all ok.