Yesterday, alongside iOS 15.6.1, iPadOS 15.6.1 and macOS 12.5.1 Apple also released watchOS 8.7.1 for Apple Watch Series 3 only. This minor bug fix release addresses an issue that could cause the device to reboot unexpectedly for some users. Even though the other OS releases address some security problems, watchOS 8.7.1 doesn’t – hence why it is only available for the Series 3.
More information can be found on Apple’s security page, which shows exactly what was fixed in each software update.
The new update can be downloaded by gong to the Apple Watch app on iPhone > General > Software Update.