In December, Order of the Sinking Star was announced. Jonathan Blow's new game that's been a decade in the making. Understandably, the focus has been on the game and its insane amount of puzzles (apparently ~1400). But the engine itself is also planned to go open-source after release. Lede is buried at the very end of this article:
"After Order of the Sinking Star releases, Thekla, Inc. will give out the custom-built game engine for free as an open-source project."
Seems to have been flying under the radar. (Or alternatively I've been living under a rock.)
No information on the license of the engine yet. Fingers crossed for a permissive one. Either way, big news.