It‘s used in Facebooks css. Remembered it from a nice article from Ahmad Shadeed. And while this limbo sure has some usefulness, it‘s way too obscure to use for the fun of it.

To add to this: CSS really has come a long way. This border-radius example can be done with Container Queries by now, which has quite good support already.

Lightning CSS is also great. A minifier at its core but also includes transpiling for older browser

Definitely not widely supported enough. Made the switch from sass back to css quite a while ago and let postcss polyfill less supported features like nesting.