If you want to use nesting, for example, you will have to give up Windows < 10 and iOS < 17.2. It's a big ask for some sites.
Safari has supported CSS nesting since v16.5. The specification was updated to remove the earlier requirement to use & though, and support for that was introduced in v17.2. As long as you include the &, you can support everything back to v16.5.
Not sure what you are referring to regarding Windows If you do need to support browsers that don’t understand CSS nesting, use PostCSS or Lightning CSS. They will transcode your nested CSS to older syntax browsers support. Then, when you drop support and remove those browsers from your browserslist, they will stop transcoding it and the CSS you deploy will get smaller. But you’ll have been writing standard nested CSS all along.