Surely if the security has changed, it would be fine, For example say that when the original vintages were released and you could go on plane's, it would be fine since technology has changed and is more advanced now. However I understand your point, but it is defiantly fine to bring them on the plane. I recently went on a plane on the 6th August when returning from Rhodos Island in Greece/Turkey. They did not say anything, id imagen with such a small device that even the biggest signals from the phone would not affect it anyway because the Tamagotchi itself is so small.
Im going to spain in a few weeks and will be taking my ID L with me. I don't plan on taking the batteries out so It should be okay anyway since it was on the way to greece.