This is the one most teams forget to ask, because it's one step removed from the vendor in front of them — it's a property of whichever underlying model provider the vendor builds on. Without a zero data-retention agreement, your inputs can be used to improve a public model, which means patterns from your data effectively leave your control even if the raw data itself isn't shared anywhere.
Ask directly: "Which model providers do you use, and do you have zero data-retention agreements with them?" A vendor that can't answer that question specifically, by provider, hasn't actually checked.