So, as I predicted, SK denied EU261 compensation:
The reason for the cancelation of flight SK0597 on 21 April 2023 was due to Air Traffic restrictions. To give a short explanation, when the air traffic is too intense, we will be allocated a departure time by Air Traffic Control (ATC). This is called a SLOT time and the airline has no possibility to influence this decision.
Delays due to air traffic restrictions are considered ‘extraordinary circumstances’ and, therefore, I am not able to compensate you according to EU261 Regulations.
Would you say this is worth challenging or should I just let it go?