Usually those who are more active in the site or those who are playing regularly or at least checking the site everyday may notice new challenges earlier than those who are relying on the email notification for the new challenges.
I believe the site sent the email after the challenge is live on the site and I think there is no other better/faster ways to get notified asap except by actively checking the site more than others so you may notice it earlier than others.
They can check their emails, check the casino website, or check the casino's social media like X.
The casino only tries to announce their marketing, programs, events to users but at the end if users don't want to receive these things, it's their responsibility. Who missed it can feel bad but if they truly care about that, they will regularly check their emails or the website or social media like they're always stay tuned for news from Lucky.fun.
Bookmark it and check it if they want, it's not too difficult.
https://x.com/luckyfun