Back when iOS 14.8 and iOS 15 were both being signed, I briefly upgraded an iPhone 7 to iOS 15 and connected it to a Mac running MacOS Sierra and iTunes 12.6.5. After accepting the MobileDevice update when prompted, iTunes 12.6.5 recognized the iOS 15 device. Backing up seemed to work, although the device had just been wiped and was otherwise empty, so it might not a robust test. I downgraded back to iOS 14.8 before the signing window closed and didn't run more extensive tests, but on the surface for Macs, it's likely that if a device worked with iTunes 12.6.5 on iOS 14, it'll also work on iOS 15.