I believe you that the dealer suggested it at 100k, but to know knowledge Subaru themselves do not suggest it in any country to service manual, and the general consensus for many automatic transmissions and finely with CVTs is that a flush is a very poor idea. There are a lot of chances a flush a could loosen debris and shavings etc and get them stuck in the valve body, sensors/actuators/passageways etc all throughout the transmission. The best advice is to just drain and refill every 30k and at 100/120k to drop the CVT fluid pan and filter and replace it with a new filter, O-ring, pan, gasket, bolts, and then refill. A repeat of the drain and fill process as I’ve explained before is also a good thing, to increase the amount of fresh fluid in the system without flushing and potentially loosening any debris.