None of the exploit chains Google found support iPhones too old to receive the latest iOS version, probably because that would require 32-bit versions of the exploits. There also doesn't seem to be any 10.0.x or 10.1.x-only jailbreaks, so that doesn't explain the ancient exploits for those versions either; there seem to have been a few months at the end of 2017 where the best jailbreak was a 10.2.1-only one, but that version ain't supported by any of the exploits here and it was quickly obsoleted by jailbreaks which supported all 10.x versions. (Interestingly, those jailbreaks used the same bug as exploit 2 here, but they supported more versions and used different techniques to exploit it. That strongly suggests this exploit was developed and used prior to the release of those jailbreaks in late 2017/early 2018.)
Also, note that the exploit used in chain 4 was unpatched when Google discovered it. It looks like the reason it doesn't support newer iOS versions is because they abandoned it in favour of the cleaner chain 5, which entirely obsoletes it in terms of versions and devices targeted. Just this pair of exploits alone suggest that the attack was live in the wild for at least a year.
In theory I suspect that most recent exploit could be backported to cover all the iOS versions covered by all the other exploits too (and some they missed) but they just didn't bother.
Semi-unthethered jailbreaks exist for 32-bit iOS devices running upto iOS 10.3.3. I wonder whether complete exploit chain as detailed in the project zero wasn't possible for 32-bit devices or the attackers just didn't care about those devices.
Also, note that the exploit used in chain 4 was unpatched when Google discovered it. It looks like the reason it doesn't support newer iOS versions is because they abandoned it in favour of the cleaner chain 5, which entirely obsoletes it in terms of versions and devices targeted. Just this pair of exploits alone suggest that the attack was live in the wild for at least a year.
In theory I suspect that most recent exploit could be backported to cover all the iOS versions covered by all the other exploits too (and some they missed) but they just didn't bother.