Many thanks for your interesting message concerning the use of copper tubing to deter slugs.
We’ve used copper rings, tape and various other copper-based to combat slugs in our own Harrod Horticultural Kitchen Garden since its inception back in 2005, and as far as I’m aware, there has been no record of any copper deposits in the soil.
I’d be inclined to re-instate the tubing but if you are in any doubt, you might want to consider some of the other organic slug control methods we have available; such as Nemaslug, iron ferrous-based slug pellets and various successful traps.
The nematodes which go to make up Nemaslug are particularly effective when used regularly from the start of the growing season but for a quick-fix at the tail end of the gardening year, the pellets and traps get the vote.
Many thanks once again for your enquiry and best wishes for your winter crops.