Earlier quoted context omitted.
You can definitely check on whether the user answered yes to the prompt, because if they declined you'll get a null (ie. all 0s) uuid. Whether app developers can rely on that as confirmation for tracking on their side is a purely legal question, and I wish the French government would try to resolve it on their side rather than going straight to fining Apple.
As a European Apple user I welcome any and all fines that can be levied on Apple for their anticompetitive practices.
The EU (through GDPR) also wants some sort of affirmative consent for tracking. That's fair, and results in one prompt. However, iOS obviously can't accept a "trust me bro" from the app itself that it's okay to enable cross-app tracking, so you need a second prompt. The obvious solution would be to combine the two, by allowing the ATT prompt to be used as consent for the purposes of GDPR. Why didn't French regulators go with this solution and decide to fine Apple instead?