Back in the day QNX used to have advertisements about having the entire OS and a GUI on a 1.44 MB disk:
* https://crackberry.com/heres-how-qnx-looked-1999-running-144...
With a friend and stimulated by the QNX demo we ported a Squeak Smalltalk graphical environment to one 1.44mb disk: http://swain.webframe.org/squeak/floppy/
Basically we use a base Linux and modified Squeak to use SVGAlib instead of X-Windows.
Later on some friends created a whole OS in Squeak (e.g. you could browse the TCP/IP protocol as a class in Smalltalk): https://wiki.squeak.org/squeak/1762
The nopsys/CogNOS project is more recent (commits in 2018): "CogNOS is actually a third iteration of the orginal SqueakNOS project ..."
https://github.com/nopsys/CogNOS
There is a conference paper about it, also from 2018: