What to do with outdated hardware in terms of Meltdown and Spectre Apple stated the following in this support article about Meltdown and Spectre: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?
Depends on what your definition of safe is.

If you want to have a guarantee that your computer will not be compromised through the Meltdown/Spectre techniques - no, it is not safe.

If you want to be reasonably sure that the risk of your computer being compromised through the Meltdown/Spectre techniques in the short term is low, then yes it is safe.

Please be advised that even if you do no install new third party programs, you will still be vulnerable through Javascript, Flash and similar on web pages, and through programs that feature auto-updating systems that could potentially download malicious software.

As far as know right know, there is a significant difference between the safety of iOS devices (iPhone/iPad/AppleTV (not first gen)) and Intel Macs. This is due to the fact that the Intel CPUs in the Mac is more susceptible to Meltdown/Spectre than the ARM CPUs in the iOS devices. I.e. more variants of exploits are successful on Intel than on ARM at the moment (considering that you the device is outdated and will not receive any software or firmware updates to mitigate these attacks).