Why not just trust people to keep their phone in their pocket when out at a social gathering that was formed for the expressed purpose of disconnecting? Wouldn't that also help the person break the habit of checking their phone frequently while also knowing it is still there?
A few years ago, I realized my screentime was getting out of control. Being a person who has a history of substance abuse (and recovery), I recognize addiction when I see it. So, I took some measure to minimize my use.
1. Remove all non-essential apps. Games, news apps, social media...basically anything that was not helping me get something done. I ended up left with email (work) any my banking or payment apps, while all the "fun" apps got installed on my tablet that stays at home.
2. Use a minimalist app launcher overlay. It's actually crazy how the removal of graphical icons helps, turning everything in lists of words, making you have to think about why you just unlocked your phone and find the specific app instead of something like "oh, there's Discord, I might as well check that while I have my phone open. These launchers also tend to have built-in blockers for notifications.
It seems like these were two simple things, but they were not. Especially at first. For about the first month, I found myself pulling my phone out for no reason, since the trigger that was making me do it (boredom, anxiety, FOMO) was still there, but I learned to cope with them and eventually they faded.
I would not have accomplished that by handing my phone off to other people, which actually makes me more paranoid about what's going given modern privacy concerns. In the US, we even campaign against handing your phone over to law enforcement since the implicit trust has been broken one too many times. Do Europeans share this anxiety?
Do people not encrypt their phones?
Of course some state actors might be able to break it with enough time but that should be enough to protect from most petty criminals.
Though I get that you might be unwilling to part with your phone. I guess you could just keep it at home.