Unless there is more to the story, you have described a low risk environment for human health. 

How close to the fence do you want to plant? Does the ground slope from the fence toward your garden bed, where runoff from the fence might go directly toward your garden? If not, you could safely plant annual crops quite close. In most soils, any chemicals from the fence will go down into the soil, not spreading out more than an inch or so laterally. Root crops (potatoes, carrots, beets, etc.) are more likely to take up contaminants, so you may want to plant those a little further away from the fence. There is no magic distance, but staying a couple feet away should provide ample safety margin in most cases. 

For the landscape timbers, most of the chemicals will remain close to where the timbers were. The preservatives are designed to inhibit the activity of fungi and other microbes that would normally break them down in the soil. As a result, they can persist for many years, bound tight to the soil. That said, the benefits to eating fresh produce greatly outweigh the marginal risk from eating crops grown where a landscape timber may have rested a few years ago. Again, root crops are more likely to take up any persistent chemicals, but the situation you have described is low risk.