This message was deleted.

Ahhh, a friend came to the rescue. In case it helps someone else:

const subnets_output = pulumi.all(
    params.subnets.map((subnet) => {
      return { id: subnet.id, az_id: subnet.availabilityZoneId };
    })
  );
  const matching_subnet_ids = subnets_output.apply((subnets) => {
    return subnets.filter((subnet) => supported_azs.includes(subnet.az_id)).map((subnet) => subnet.id);
  });