I'd say that the archive of the Pebble watchapps and watchable before pulling the plug are fair game and should be made available to Core.
It's all available here too https://github.com/aveao/PebbleArchive
Anything modified by Rebble after that point is up to the Rebble team to decide if they're willing to share. My only wish is that these changes are publicly available for non-commercial use, otherwise it's just another walled-garden.