Treble's compatibility system isn't very relevant to us right now. There's a new Android release every month: a monthly, quarterly or yearly release. The devices we currently support (Pixels) receive each of these updates officially. Most Android devices do not get the monthly or quarterly updates, only the yearly ones. Other devices rely on partial backports of security patches (Android Security Bulletins) to an initial release, which are provided for ~3-4 years after the initial yearly release. If we supported typical Android devices with that situation, then we'd at least partially rely on Treble to provide the latest OS version. Pixels are currently the only devices meeting our hardware security requirements listed at https://grapheneos.org/faq#future-devices. Having proper updates for 7 years from launch is just part of that, most of the requirements are for hardware-based security features like the secure element features, hardware memory tagging, pointer authentication, USB controller support for blocking new connections and disabling USB data, etc.
GrapheneOS uses the 6.1 and 6.6 long term support branches of the Linux kernel with 6.12 as the next one that's likely going to be used to replace 6.6 for on-device virtual machines and the emulator with Android 16.