[jinushaun]

The new FileProvider API supports something like smart sync: placeholders for files that live in the cloud and download on demand.

[Mister_Snuggles]

OneDrive does this without requiring a kernel module.

[hemancuso]

They have access to a kernel module and entitlement that only they have access to, com.apple.fileutil - and Apple has said they aren't giving out any more entitlements for it.