The Swiss Army Knife keyring was a replacement for the shackle or bail attachment mechanism, that was used on many of the older Swiss Army Knives.
The 'keyring' typically consists of a small metal loop, on a spring or liner, to which a small split-ring is attached. The split-ring provides a larger attachment point that is suitable for a lanyard, or to connect to a regular keyring. Wenger often attached a small chain with split-ring to the metal loop on many of their 85mm models.
Between 1964 and 1992 Victorinox used a hollow rivet as an attachment mechanism on some Alox models. The rivet goes right through the whole frame of the knife, which allows a lanyard to be attached, by threading it through the rivet. Wenger continued to use this mechanism in their Soldier model until they no longer made Swiss Army Knives.
On many of the non-Alox models, the keyring transitioned from being a looped extension of the liner by the blade, to being a looped extension of the spring of the opener layer, around 1991. This can be a useful reference to help date medium sized Victorinox SAKs.
Models with a USB flash drive have a split-ring attachment point built into the plastic flash-drive cover / release spacer.
Victorinox creatively include the keyring as a tool when counting the number of functions of a SAK!