I have a waterline for my present fridge ice maker connected, but it hasn't functioned for several years. Now I'm going to be getting a new fridge with a factory ready ice maker. The existing line is a copper tube that goes through the wall and then through a small hole through cabinetry. Then through a small hole at the other side of the cabinet to the refrigerator. The pipe where the cold water originates is not accessible. Should I cut out the cabinet hole much larger and also the drywall just in case there is a leak or is this a common setup and I shouldn't be concerned?

In videos I usually see that the refrigerator is near the kitchen faucet so the ice maker line comes from there, and is accessible. Thanks in advance for any suggestions

The shut-off valve is usually where the copper tube connects to cold water piping. In your case it is now inaccessible. You probably don't have a shut-off valve at the other end of the copper tubing.

So you will have to turn the water off to the whole house in order to be able to disconnect your present fridge and connect the new one.

Copper tubing is very reliable and leaks from the tubing itself is very rare. The only place where it might leak would be the joints at the end of the tubing and, if the joints were made properly, would be unlikely to leak. So I would not bother cutting holes at this time.