I would not worry, we graze sheep in fields that have had pigs in them all the time.
Copper is essential for life.
Sheep need 5mg per kg of copper in their diet. Levels above 25 can cause problems - symptoms are that sheep go lethargic, drink lots and grind teeth. Untreated death can occur. A drench from the vet can cure/aid recovery if promptly administered.
Levels of copper can be higher in pig grower feeds, as it can aid growth performance. Of 5 labels we have from different feeds we have used over the years, one shows 17mg per kg, two 25mg (inc one grower), and one grower 171mg!
Feeding any of these daily directly to sheep would be likley to create a problem!
However any residuals in the soil should be slight - pigs need copper because they use copper, and as they grow and go to slaughter they will take much of it with them, not leave it in the soil.
Sheep don't eat soil, so this copper would need to transfer from the soil into the grass at a rate above 25mg per KG of grass, which I would be highly doubtful would happen.
Have just read you sheep thread about these being highland things, ours are lowland, but am still sceptical that grass would soak up sufficient levels to be of concern.