BIKE FACT: Your inner cable doesn’t stretch!
The outer cable on your bike actually compresses, that’s why your gears need a tweak. The inner gear cable is pre-stretched to a few tonnes during manufacturing, however, yes, it could still stretch but not with the amount of force you can generate through shifting gear.
Over time, the outer actually needs to be trimmed to create a fresh end when you get a service. This is also why you may have to replace the outer after a few services, as they can only trim so much off before the outer becomes too short to fit the bike.
There are a few companies that manufacture an outer cable system made from small aluminium sections that fit together to make the outer, these won’t compress. Once your gears are set they will be set.
I’m not sure why bicycle shops tell customers that the inner stretches, maybe because that is the part being replaced, so it leads to peace of mind for the customer and less explaining for the mechanic.