If you make mistakes, just backup your wallet.dat again, your old backup wallet.dat will expire and can't to use again
Bitcoin doesn't work that way. Your backups NEVER expire, anyone telling you that is lying. When you change/encrypt your wallet, the keypool will refresh (or the seed will change) but your previous addresses will still be inside and anyone with it can access them. So, you might end up losing your coins. When you suspect that your wallet is compromised/you threw a HDD with an unencrypted backup, encrypt your wallet and send your funds to a newly generated address.
what he meant by "will expire" might have related to how non-HD wallet behaves.
expired in the meaning the first backup for the first 100 receiving addresses would be irrelevant.
after the user has received his transaction on 120th receiving address and forgot to create new backup,
he could risk losing his access to the next 100 generated address (address #101-200) in non-HD wallet
or... I, too, misunderstood what Quickseller said below
With a 'traditional' wallet.dat wallet, you will need to backup your wallet at least once every 100 transactions, or else you will risk losing access to some of your funds, and the process of backing up your wallet is not without risks.