iBeacon/CLBeaconRegion is in fact still missing from the watchOS 4.
On iOS, Core Bluetooth excludes any iBeacon data from scanning results, the only way to use iBeacon is via Core Location. We’re yet to test if this is the case on watchOS 4, or given the fact that there’s no iBeacon on Apple Watch, maybe Core Bluetooth can be used to detect the iBeacon packets, unlike on iOS.
And in any case, you can always detect non-iBeacon packets via Core Bluetooth on watchOS 4. For example, Estimote Telemetry, or Eddystone packets.