Most production-ready obscure free OS outside embedded?

Haiku has made great progress, Plan 9 is still active in some forks (e.g. 9front).

Helenos (http://www.helenos.org) might also be worth a look, especially since it's a modern microkernel-based system.

Minix, if you want to compile text-mode programs as if you were batch processing the month-end invoices on a minicomputer in the year 1983.

OpenVMS, if you want to resurrect some actual software that was used to do scientific calculations on a VAX in the year 1983.

c64 / 6502 hardware[1] seems to get 'new' os & updates, although lot of diy hardware [2][3]. Tensorflow light even has a C64 port![4] Although, might want to use it on the fpga hardware and not the original c64 hardware.[5]

Wikipedia has quite an electic collection of free software os systems[6] and microkernel based os's[7]

Although, using a bootloader / webedge[0] combination does allow for running any of the [6] and [7] os'es.

[0] webedge : http://github.com/WasmEdge/WasmEdge

  ( http://github.com/skiffos/SkiffOS 
    http://medium.com/@raphael.outhier/joys-of-porting-a-kernel-to-webassembly-5f0124e71080 )

[1] : http://www.c64-wiki.com/wiki/Operating_System#picoOS

[2] : http://www.geeky-gadgets.com/build-a-commodore-64-16-01-2023...

[3] : http://www.hackster.io/news/building-a-brand-new-c64-96e803e...

[4] tensor flow light : https://hackaday.com/2022/07/06/tensorflow-lite-on-a-commodo...

[5] c64 fpga : https://github.com/MiSTer-devel/C64_MiSTer

[6] : free os systems : https://en.wikipedia.org/wiki/Category:Free_software_operati...

[7] : micro-kernel based : https://en.wikipedia.org/wiki/Category:Microkernel-based_ope...