Apple has just released iOS 11.4.1, HomePod 11.4.1, tvOS 11.4.1 and watchOS 4.3.2 to all eligible devices. There is detailed release note only for the iOS update, which mentions the USB Restricted Mode. The new feature does not allow third parties to break into the phone without the owner’s agreement, effectively tightening security even more.
All four updates count as maintenance releases, even the iOS one. It can be installed over iTunes or OTA via Software Update feature.