Once it was popular to have a cord from knife to belt so it didn't get lost. As a young boy I did this Scout style.
They can be useful as a pull cord or tag. For pulling a folder out of a deep pocket or deep belt sheath.
Bushcrafters have found all sorts of ways to keep a big heavy blade in the hand when chopping. Think its the love of paracord.
I find lanyards a liability on the whole. When working they get in the way and are annoying and a distraction. Something to pick up dirt and germs too. Wet conditions they are just another wet thing, dry conditions not so bad. They catch on things and a fast way to lose a knife plucked from its place. No one puts a lanyard on a hatchet so why on a big knife? In truth I think long lanyards are an accident waiting to happen, and a quick way to get a finger ripped off from the cord catching on machinery. When a tree or branch is felled or cut anything can happen and kick back is a worry; don't want anything attached to the hand. Short stubby pulls aren't so bad. If I fell when using the knife I'd want to get away from the blade, not have it attached and follow me down. However, if working at height then no reason not to have a safety cord, but a temporary one can be done for that.
As for fixing a bright coloured flag then why not just get the handles bright, heck dip the end in bright paint. In deep snow there is some good argument to have a long ribbon, bit of a pain though if its going to be long enough to be of any help; resheath the knife instead which is what should be done.
Your call, but generally I don't use a lanyard. Nothing wrong with having the hole there if ever needed.