The bootloader software has now been updated to include new versions or Ubuntu, Debian, Manjaro and Mint. Anyone with a key can run the update procedure to download new images.
More details are available on the MadPenguin support forum.
Anyone who wants to purchase a key can do so through the Mad Penguin shop or directly via Forums.
Note that when it says “subscribe”, it’s just “subscribe to a single payment”, nothing more.