Luckily it’s DRM-free. Back up your installers. I wanted to call attention to this, because in a very unusual move, it’s being removed even for people who own a copy, whereas usually stores will only remove a game from sale and still host the files for existing owners to download.
I only own the game through itch.io, which I got through one of those charity bundles, and they contacted me by e-mail. Then people contacted GamingOnLinux about it, and at least right now, it seems to only be limited to itch.io.
EDIT: I actually do have the game via GOG as well, which is news to me, and I did not receive a similar notification about it.