That's the common belief, but it might simply not be true. Two reasons.Don't ever buy a used mining card. Electronics do wear out from heat.
Miners tend to run their cards as efficiently as possible, rather than as highly clocked as possible. Since the efficiency curve drops like a rock the closer you are to max clocks, miners underclock and undervolt their cards to an optimal point. This means that despite the card running 24/7, it is running in a less "strained" state than if you were gaming. I do get that mining uses more of the GPU at any given time and generates more heat, but I don't see how that's an issue, because...
Changes in temperatures are more damaging to hardware than high temperatures (within reason of course). If you run constantly at 70°C for a year, or if you cycle between 35°C and 70°C every day for a year, the latter will fail much earlier than the former due to shrinking and expanding of the components.