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Open Firmware: https://en.wikipedia.org/wiki/Open_Firmware

OpenBOOT: https://openfirmware.info/OpenBOOT

That second link has a link to a git repository and you can see the forth code there.

The original author of (that first implementation of) Open Firmware, Mitch Bradley[1], is still active on GitHub and in particular in Forth-specific discussions, by the way.

[1] https://github.com/mitchbradley

I recall a RedoxOS developer mentioning they were using a FORTH in the bootloader or some other very low level piece of that project.

FORTH is the type of thing that probably exists all over the place but it's so deep and arcane that you would never know it.

Last I looked FreeBSD was using FORTH in their bootloader

well there is https://collapseos.org/ :)

Bitcoin’s script language for smart contracts / spend conditions is Forth.