I asked a question yesterday about brake blocks for plastic wheels.
At the time it seemed on-topic because its totally bike components. However not all scooters are built with bikes.
This seems to be against the trend of other answers, but I would NOT allow them.
If we allow scooters, we are on a slippy slope and where do we draw the line? Do you also allow skateboards? Roller skates? Skis?
For me the minumum requirement should be: "must have wheels and pedals" with one clear exception for balance bikes. This includes trikes and uni's whilst ensuring it is something that gets cycled.
Fundamentally the cycle part is why motorbikes are off topic
I would also vote against at least certain kind of questions. In term of use, they somehow compete with bikes (mostly electric shared bikes though), but they are ultimately another class of vehicle.
About the inclusion of the category because some use bike components, it's unlikely that a question asked is actually dependant on the vehicle on which the component is mounted. If a scooter uses bike brake calipers, it's likely that it will be relevant for bikes as well, and should be on-topic (because it's about the component, not the vehicle).
For the rest, the small-wheeled e-ones are being developed from different point of view than bikes: it's an integrated package where it would be hard to have the same kind of questions that we typically see here.
Perhaps kick scooters are something like 30-40% on-topic for this SE site. The exact percentage doesn't matter as much as the contentions that:
Kick scooters should be on-topic. Bicycles stack exchange is not limited to the narrow definition of two-wheeled, single track human pedal powered vehicles. We already have questions about unicycles, tricycles, and handcycles. Unicycles are one wheel short of being a bicycle, tricycles have one bonus wheel compared to a bicycle, handcycles are not strictly bicycles either, and kick scooters lack pedals. But as a community, we should welcome kick scooters as part of the wider family of variations on the bicycle that we can see on the road.
Source: Wikimedia Commons
I created the kick-scooter tag to cover such questions.