I noticed Xcode 9 now always generates an Assets.car file regardless of deployment target for macOS apps. This wouldn't be such an issue except I have a lot of (large) icons and now my bundle size has doubled with no real benefit. I could see generating the CAR if I had other resources like PDFs, image sets, etc, but I don't, it's just an app icon and a bunch of file type icons, and now my app bundle has basically two copies of every image. It seems like High Sierra can use an Assets.car alone, but since I deploy back to El Capitan (10.11) I need all the ICNS files too, which seems like a waste.
Checking the actool(1) man page, it looks like you can control the minimum deployment target, but not the "maximum" one or some other way of constraining the output. It may seem like a small issue but the extra files are just a waste for me. I'll be happy to have the single CAR file when I raise my target to 10.13, but that's a long way away.