> I’ve only ever seen it done in pizza places that already have takeaway boxes available.

Most, if not all, restaurants have something they can drop leftovers into if you ask for it.

As someone who lives in Spain but is Swedish, I've never had any restaurant tell me "we don't have takeaway boxes" or "no, we won't do that" when asking to take my leftovers with me, neither in Spain or Sweden or any other country I've visited.

Pizza places are also one of the few places in Europe where the portion is often too large.

Otherwise I think the rare occasion where someone requests it is when a younger child has hardly touched their meal.