Just some historical notes:
OS X 10.0, 10.1, and 10.2 did not change the system requirements. These were "rapid release" OSs, with 10.1 coming 6 months after the horribly buggy and mostly unusable 10.0(it was a free upgrade) and 10.2 coming 11 months after 10.1.
10.3(Panther) came out 14 months later and dropped support for "Old World ROM" Macs(those without built-in USB). This was somewhat of a "soft" limit and Panther is easy to make run on OWR computers.
10.4(Tiger) came 18 months later and dropped support for computers without built-in Firewire. Once again, this was a "soft" limit, and in fact with minimal work it can even run on 603 and 604 processors if you don't mind waiting a few minutes for a program to launch
. The only real limit is in RAM.
10.5(Leopard) came a full 27 months after Tiger. Officially, it dropped support for G3s and for G4s slower than 867mhz. Unofficially, the G3 limit is a "hard" limit while it will run on any G4 with enough RAM. The work-arounds for this depend on the computer-on all but one production G4, all that's needed is to defeat the speed check on the installer. Computers without AGP graphics need some additional work, but it can be made to run on most computers as far back as PowerSurge Macs(90s beige boxes) with a G4 upgrade. One of these days, I'll get it running on my 8600.
10.6(Snow Leopard) came out 22 months later, and dropped support for all PPC Macs. This is a hard limit.
10.7(Lion) came 23 months later, and dropped support for 32 bit(CoreDuo) Intel processors, the very first Intel Macs. This is a hard limit.
10.8(Mountain Lion) came out 1 year later and dropped support for computers with 32 bit EFI. This covers several 2006-2008 era Macs, but is a soft limit.
10.9, 10.10, and 10.11 pretty well established the 1 year cycle we are now use to. These three versions kept the same system requirements as Mountain Lion. With that said, increasing GPU demands meant that most computers hacked to run Mountain Lion couldn't realistically run any version past that. The notable exception is the Mac Pro 1,1/2,1, which could keep trucking on with a new graphics card.
The change in system requirements for 10.12 was not unexpected. Many computers that were "cut off" like the MBP 3,1/4,1 and the later MacBooks can be made to work, although sometimes at the cost of USB and/or Airport. There is also a "hard limit" in needing SSE4 support, so the long-lived MP 1,1/2,1 can't be made to run it.
High Sierra honestly cut off very few computers that Sierra supported, and I suspect that most of the computers that could be made to run Sierra can also run High Sierra.
We'll see what 10.14 brings, but we may get another round or two of "complacency."