You can run Snow Leopard on a Late 2011 Macbook Pro which shipped with Lion. However, you must run at least 10.6.7 I believe. I've done it out of curiosity.Even if you would restore a working OS < pre-installed OS (= macOS 10.12.5 "special build') it will not start up.
All necessary drivers simply exist only on the pre-installed OS and upwards.
Has been like this on Macs since.... ever..?
Yes, that's the reason.On an iMac it will be tough due to GPU driver support. Not saying its impossible, but it is impractical. And if its not a "fun project" will likely end up being a "total nightmare".
I'm assuming you have some software that doesn't have Sierra support?
Looks like I shouldn't even try it then.I couldn't even run the usual release version of 10.12.5 on the 2017 iMac... and the 2017 iMac ships with 10.12.5. The Mac refused to boot.
10.12.5 on a 2017 iMac is a later build than the release version of 10.12.5 that all the other non-2017 Macs have. Thus, I could not do a direct clone from an older iMac with 10.12.5 to the 2017 iMac.
This problem is solved now though, since 10.12.6 is out, and it should be the same build for everyone.