A question in a Facebook group about a GPDSO for an IC-9700 to be used on 23 cm for FT8 inspired me for a quick test. As I use my IC-9700 for 2 m and 70 cm only from home, I connected a dummy load to the 23 cm antenna socket. Next was to tune it to 1,296.174 MHz (of course) and to call in FT8.
The result in brief: No problem to decode, a moderate drift and the signal appeared about 100 Hz too low. Nothing to worry about.
But never the less, I prefer JT9f on 23 cm, as FT8 is useless on aircraft scatter.