It seems he is talking about any Bitcoin client, not only Bitcoin Core, since Electrum also has a backup file (defaultwallet).

That's why having a backup of this on a cold storage device is way better because you can use it offline to sign a transaction without upgrading it to the latest version.

So if any of these Bitcoin clients you're talking about ends support of the old wallet, then having an extra backup of these wallet files can still be recoverable, or dumping it's private on a cold storage device is a great idea and save them somewhere safe.