The official answer from us is that we don't comment on rumors or speculations.
graphox Will GrapheneOS be compatible with Pixel 9 too?
However, in this case, I don't see why it wouldn't be compatible?
Or "when" will the list of "supported devices" be updated after a new Pixel phone has been launched?
When it's been thoroughly tested and ready.
Pixel 9 presumably will be launched in October 2024.
Perhaps, you've already answered your own question for the most part.
You're welcome to do your own homework and compare the initial release build of new devices added to our list against the launch dates of those devices.