Apple stated the following in this support article about Meltdown and Spectre:
All Mac systems and iOS devices are affected, but there are no known exploits impacting customers at this time. Since exploiting many of these issues requires a malicious app to be loaded on your Mac or iOS device, we recommend downloading software only from trusted sources such as the App Store. Apple has already released mitigations in iOS 11.2, macOS 10.13.2, and tvOS 11.2 to help defend against Meltdown. [emphasis added] Apple Watch is not affected by Meltdown. In the coming days we plan to release mitigations in Safari to help defend against Spectre.
Is it safe to use hardware that cannot be updated to one of these three operating systems? Furthermore, in the case of obsolescent devices, are there significant differences in the safety of old iPhones, Macs, and Apple TVs in terms of these vulnerabilities?