[st_goliath]

Microsoft Research had an experimental OS project at one point that does just that with everything running in ring 0 in the same address space:

https://en.wikipedia.org/wiki/Singularity_(operating_system)

Managed code, the properties of their C# derived programming language, static analysis and verification were used rather than hardware exception handling.

[avadodin]

Fil-C vs CHERI vs SeL4 vs YOLO

I think hardware protection is usually easier to sell but it isn't when it is slower or more expensive than the alternative.

[Joker_vD]

"Operating System Principles" (1973) by Per Brinch Hansen. A full microkernel OS (remake of RC-4000 from 1967) written in a concurrent dialect of Pascal, that also manages to make do without hardware protection support.

[alnwlsn]

I think TempleOS also worked like this, though its certainly better known for its "other" features.

edit: I missed it was linked on the above page

[rep_lodsb]

In TempleOS, everything runs in ring 0, but that's not the same as doing protection in software (which would require disallowing any native code not produced by some trusted translator). It simply means there's no protection at all.