I'm pretty sure I know the answer to this, but it's worth pointing out and I'd be interested in the reasoning behind the changes. In all previous OS X versions, it's been possible to use setxattr() (as root) on /Library/Application Support. This seems to be no longer supported under Sierra, giving a "not permitted" error. It is also no longer possible to even "sudo chmod a+w" on Application Support (same error).
Can someone explain if this is in fact a behavior change in Sierra? I can't find any relevant documentation to back this up.