They say your transaction history with Google Pay won't be used in other Google services. How much you trust that is up to you.
It is only change in "terms of service" away after enough people are using it.
The Google Rewards app is always wanting a photo of receipts when I pay with a credit card. Why would this be any different, especially when they would have access to the data so easily?
I do want an entity I trust at a given time (eg. Google, Apple, some credit union, etc.) to mine the raw data they have on my account and do something useful with it as long as neither the raw data nor the derived one is shared with other third parties.
My idea of something useful here isn't ads per se but currently I see ads for Google Fi in Gmail even though I'm a subscriber. That's pretty dumb
So one might only have hours to react before implicitly agreeing by failing to opt out or cancel. Perhaps even less of the change in terms is effective immediately.
"Do the right thing."