No dietary requirements - we have checked.
They've been doing this for years (we see them every year or two) - so it's not COVID related, though they're currently very anxious about COVID.
They know full well that absolutely everything is being bought in (they've seen the order form) and served on paper plates this time, so their previous excuse of "saving us the bother" has evaporated. Hygiene is also not a logical concern for that reason.
Instead they'll be turning up to a family event with all their food in Tupperware, and no reason has been offered.
AIBU to think it's rude to reject your host's catering? I just don't get what they're playing at.