User threading example

[VintageOSFan]

We're going over threading models in my CS class. Are there any examples of operating systems that use exclusively user threads (N:1)? It looks like 1:1 is far and away the most common model but I wanted to see live examples of this other model. Google had been inconsistent and it seems a lot disagree on which actually is user thread based or not. As best as I can tell it hasn't been used in years, looks like Mac OS 7-9 might have?

Any programs or languages that utilize user threads count too.

Any and all help is appreciated! Thank you!