The feature is part of our new T&Cs, so new customers give their consent when opening an account.
If you already have a Tomorrow account, you will get an opt-in option for the footprinting feature (Profile → Manage account → Activate footprinting). Once activated, you'll see the emission value for all future transactions right next to the purchase amount - for all past transaction, you need to tap on the booking to get to see the footprint.
You have the option to switch your CO2 footprint on and off in the settings at any time. Once switched off, your data used until then will be deleted with us and Ecolytiq, and of course no further data will be collected for the CO2 footprint feature either.