jadfnj Hi there, and welcome!
GrapheneOS can only fully provide security updates to a device provided that the OEM is releasing them. When an OEM is no longer providing security updates, GrapheneOS aims to provide harm reduction releases for devices which only have a minimum of 3 years support. Extended support updates at minimum will be done until the next Android version. It is likely that we will make a decision around harm reduction releases for other devices with longer lifetimes in Q4 2024. Harm reduction releases do not have complete security patches because it's not possible to provide full security updates for the device without OEM support and they are intended to buy users some limited time to migrate to a supported device.
Basically, after official support ends for the device, it will likely stop receiving extended support releases when the next Android version is released. However, as noted above, you should assume your device is insecure the moment official support ends from the OEM (in that case of Pixels, that's Google) because GrapheneOS cannot ship firmware updates and doesn't want to mislead its users by marketing something as still being secure when it isn't.
As a general recommendation by the project, nothing below 6th of 7th gen is recommended right now, because with 6th gen, the lifetime of Pixels was bumped from 3 years to 5 years. You'll be getting much more bang for your buck by getting one of those.
I recommend taking a look at https://grapheneos.org/faq#recommended-devices for more information.
I hope that helps. if you need anything else, just let us know!