Oh, they're not gone -- still very much part of APFS. You can read the contents of the resource fork for a file at path `$FILE` by reading `$FILE/..namedfork/rsrc`

The resource fork is still how custom icons for files and directories are implemented! (Look for a hidden file called `Icon\r` inside any directory with a custom icon, and you can dump its resource fork to a `.icns` file that Preview can open)