We drop devices that are no longer supported by OEMs for the provision of firmware/driver security updates. Especially after we exhaust our ability to provide harm reduction between their support ceasing and the next major OS update through our Extended Support Releases (only available to allow people to upgrade their devices).
We do this as we believe in genuine security and privacy provision and not using the terms as a marketing gimmick.
3rd Gen Pixels are end-of-life and are highly insecure. They do not have the most important security patches regardless of OS choice. LineageOS, CalyxOS, etc. really don't properly communicate the severely of how insecure these devices are to users.
It's not simply about having better security features on the newer devices. End-of-life (EOL) devices lack half of the privacy/security patches. They're vulnerable to known remote code execution flaws and other issues. You can't really have privacy and security on such a device.