It’s been a bit since the last game got delisted from GOG.com with a short heads-up notice. The latest is 2017’s Immortal Planet from solo developer teedoubleuGAMES. The announcement briefly confirmed that it will be leaving the DRM-free platform on Monday, January 11th by 2pm UTC. Anyone who owns the game can still re-download it and anyone who missed it can pick it up on Steam. No reason was given but as it is the dev’s only release on GOG.com it would seem to no longer be worth their time/energy/expense to keep it available.
Thanks to @Dimchkin for pointing out the news. I’ve added the date to the Watch List calendar and will have a page up on the site soon.