Here is the email BA sent on 15th July 2021 (the day I departed Chicago / 1 day before affected flight) :
Thank you for travelling with British Airways.
We would like to inform you of a change to the departure time for your flight BA2696 London Heathrow to Faro on the 16th July. This is due to operational constraints, we have had to make this adjustment to our schedule. Your new time of departure is 15:00.