Honestly I'd think about several "worse" companies to annoy with such "a test" first than Prusa, but anyway > A company can list an email address as their off

Honestly I'd think about several "worse" companies to annoy with such "a test" first than Prusa, but anyway > A company can list an email address as their official GDPR contact in their privacy policy, and if their own spam filter eats your request, it legally never happened. There is no obligation to check. There is no obligation to ensure delivery. The burden is entirely on you to prove they received it. That's you being silly. The correct way is to send a letter with a Reception Receipt https://en.wikipedia.org/wiki/Avis_de_r%C3%A9ception https://en.wikipedia.org/wiki/Avis_de_r%C3%A9ception which is the way lawyers like to do