Received a White Company gift voucher for £100 some years ago as a wedding gift for use in-store. It had no expiry date and so, with no stores near us, we kept it for a later date; assuming we could use it any time. Covid came along, my wife became wheelchair-bound & less mobile. After a house move we finally found ourselves near a White Company store so went in with our voucher - but they refused to take it, saying that "yes, the voucher didn't have an expiry date, but they'd decided to introduce one and so we'd lost our £100". A worthless dialog with customer services followed; they apparently published details of the new policy in store (unseen, of course, as we had none near us) and online. So it seems they've taken the £100, and given nothing in return .. now, hang on, there's a word for that??

Update: Don't be fooled into thinking I got a positive outcome from the White Company after a few DMs. I got a standard letter saying they'd been through a legal process and given due notice of their decision to expire the vouchers .. in effect "sorry, but you've lost your money".
Well, at least the owners will be pleased to have my £100 contribution to last year's £31.9m profit, and the £50m dividend they were able to pay themselves.