Where can I find the highest quality WRC onboards from recent years?

[flat_right]

then it will not be HD

compression doesn't define if it is HD or not. Raw video or h265, both are in HD (if filmed so) but latter limits color depth, averages (sub samples) color, and throws away information to reduce the space it takes up.

As said here, use some screen record (kazam in linux for example can record in mp4 (h264)) or there are programs available which can be used to download all live onboard files from wrc site.