The whole NeXT GUI widget set, as well as Rhapsody GUI widget set, and several 80‘s style NeXT tool icons, are still in Mac OS 15 in some assets.car file. And before car files where a thing, it must have been either TIFFs or PDFs.

Here someone even hacked an older version of Mac OS X to actually use the NeXT and Platinum styles: https://mastodon.social/@stroughtonsmith/110708615280659758