I just got a used bike with a Regina CX/CX-S 7 speed 12-23 freewheel that has a Sachs/Sedis chain. The chain could be 2 years old or 10. I don't know.
Does it matter what brand I get to replace it?
It's a Campy double in the front and Campy derailleurs.
In short, I wouldn't worry about your chain unless it is
worrying to you. If it's working OK, just keep it clean
and properly lubed and unless you ride your bike very hard
and put alot of stress on your chain, it should outlast
the other components in your drive train. When those other
components need replacing, then change your chain too.
After 2-5000 miles chain stretch becomes noticeable. Stretched chains will increase wear on freewheel cogs and chainrings. They will also eventually slip over cogs that don't show the same wear pattern, which may be why you aren't noticing any problems with 20-year-old chains.
Typically, expensive chains sometimes shift more smoothly and are lighter and often less durable than many cheaper chains. And modern 10-speed chains are less durable than the old 5-speed and 7-speed chains from the days we used freewheels.
But don't fret. Any 8-speed chain will work fine with a 7-speed cassette or freewheel.