Can I delete /Library/Updates on EI Capitan
Solution 1:
Theoretically, there should only be updates in that folder until you actually run the updates, then they should be cleared - so my first suggestion would be to check the App Store app & run any pending updates from there, reboot, then recheck the folder.
Anything left over at that point ought to be safe to delete, though I would retain the index.plist PPDVersions.plist & ProductMetadata.plist files, if present, unless you are having general updating issues, as I suspect they are designed to accurately track the update state.