In short: Slate itself collects nothing. Everything it reads — time, date, battery, solar level, step count — stays on your watch. The only outside connection is the optional purchase check made by KiezelPay, the third-party service that handles Slate's one-time $0.99 unlock.
On your device
Slate is a watchface. To draw the time, date, and gauge, it reads a small set of values directly from your watch's built-in system APIs:
All of this is read locally, used only to draw the face, and never leaves your device through any code Slate itself runs. There is no account, no analytics, and nothing sent to a server on Slate's behalf.
The one exception
Slate is free to install. Unlocking it permanently is a one-time $0.99 purchase handled by KiezelPay, a third-party licensing service used by many independent Connect IQ developers — Slate's developer does not run payment infrastructure directly.
To check whether your copy has been purchased, the KiezelPay software built into Slate periodically contacts KiezelPay's servers through your phone's connection. This check is handled entirely by KiezelPay's own code, not anything written for Slate specifically.
If you choose to buy, that transaction happens on KiezelPay's own website, not on your watch or phone through Slate. Slate's developer does not receive your payment details, and does not receive your name or email address unless you separately contact KiezelPay for support. See KiezelPay's FAQ and their End User Terms for how they handle purchase and payment data.
For clarity
If this changes
If Slate's data handling ever changes — a future version adding a feature that needs new permissions, for instance — this page will be updated and the date above will change accordingly.