Kotlin Native works on the Raspberry Pi 2 (via cross compilation). Is the latest Raspberry Pi (3 Model B+ - https://www.raspberrypi.org/blog/raspberry-pi-3-model-bplus-sale-now-35/) supported as a target?

we never tried, but generally RPi target is just linux_armv7 with hard floating point, so it likely will work as well

If a arm64 Linux distro is running on a Rpi 3 would Kotlin Native still work? The Rpi 3 (both Model B and B+) uses a armv8 CPU, although if a arm32 Linux distro is used then a armv7 CPU is listed (https://raspberrypi.stackexchange.com/questions/77693/enabling-armv8-on-raspberry-pi-3-b).

assuming proper options in the kernel (https://stackoverflow.com/questions/34648832/can-old-arm32-binary-files-be-run-on-aarch64-kernel) - yes

Would make sense to defer linux_arm64 support until there is a lot of armv8 based hw that can properly utilise the architecture, and demand starts to take off.