Nitpick: Can we please stop saying "free-to-use" and instead say, for example, "paid-with-data"?

"Paid with data" sounds nominally accurate but misleading.

Suppose there were a paid mail service, and another free one with a billion users, and the paid service's spam filtering relied on the free one for accuracy. (Spammers can't very well spam a hundred thousand people without hitting at least a few thousand of the billion.) Would you say that the users of the free one pay with data? They do, but to me, the phrase "paid with data" suggests other data than just the spam you get and whether you click "this is spam" on a particular message.

Or "spyware"? The fact that it historically meant something else doesn’t make it less fitting in my opinion.

But that implies you don't (also) pay with data if you pay for the service, which is often not true

It's not always true. Many things (often smaller things) are free to use and not paid with data.

Nope.