The primary concern in bringing an e-scooter on a plane is its battery because of the risk that it may spontaneously overheat and catch fire or explode. There are three main categories for international flights:
100 Watt-hours or less—Generally, airlines allow a battery size of 100Wh or less, as they are considered "small" and pose a lower fire risk. This refers mainly to smartphones, laptops, and cameras.
Up to 160 Watt-hours—In such cases, you have to call it in and ask for permission from the airline. E-scooters with such a watt-hour are hard to come by, and even if you find one, they'll most likely be for children.
Over 160 Watt-hours—it's a no-go. This excludes most e-scooters, as batteries have around 250Wh or way more on average.