For health (which we're talking about), life, or car insurance (since a bunch of UK insurers also have "safe driving" apps that earn you points, etc.), yeah, sure, why not?

Because obviously they can use those to profit by dropping eg people they diagnose (or just infer statistically) with serious problems before they develop, or raise their prices, etc.

This is not about some extra win-win motivation to be healthier...

It's about turning risk (which is supposed to be what insurance companies are paid for) into rent-seeking...

If the insurance scheme is public, incentivizing individuals to remain healthy in order to lower the cost for everyone is a civil duty.