When I run lightningcss --nesting --bundle --minify --targets 'last 1 year' styles/styles.css -o out/assets/styles.css, it output nested css.
With last 2 years it works. I am a bit confused what happened in the background here.
You can find an example that fails here: https://github.com/cedeber/korros-ui/blob/f6440ef367e68278bd25866870854d48a9254305/styles/trigger_button.css