As far as I can see, the T440p is the latest Thinkpad to support Coreboot/skulls.
If I wanted something newer (say, something from 2019-2022 or so) under $400, what could I even get? I want to run my own choice of linux distribution on it, so most chromebooks are out of the question (also I'd like something more powerful and upgradeable).
coreboot, formerly known as LinuxBIOS, is a software project aimed at replacing proprietary firmware (BIOS or UEFI) found in most computers with a lightweight firmware designed to perform only the minimum number of tasks necessary to load and run a modern 32-bit or 64-bit operating system.