IT'S COMING!
https://grapheneos.social/@GrapheneOS/113347818412770072
Stable channel release of GrapheneOS based on Android 15 will begin rolling out to devices. To avoid the servers being overloaded due to much larger than usual deltas, we'll be starting with the newest devices and working towards the oldest ones. Can use Beta if you want it now.
According to the Releases page it's already on stable Pixel 9 as of today.