My office's 5 ancient 2009 spec iMacs are on their last legs and I am going to have to replace them in early 2017 with the new models. I know that these latest iMacs cannot run any OS older than they ship with - which rules out downgrading - but does that apply to a Yosemite VM too?
I have some plugins that won't load into Sierra based installations (have tried on a non critical machine here) and there is no upgrade path due to the version we are running so I'd have to stump for a full copy on each machine that I run here - mucho expensive!
Would a Sierra host OS run a Yosemite OS guest in Fusion 7.5 Pro?
Running an earlier OS X version like Yosemite as a guest OS within VMware Fusion on a host that runs macOS is one of the normal use cases and does work pretty well.
You didn't mention what type of software the plugins are for. There's a few use cases that a virtual OS X is not so great and it originates in there not being 3D support on OS X / macOS guest OS's. So if you are depending on specific features there then it might not work.
Thanks for the speedy reply. These plugins are all for Apps within the Adobe CC 2015/17 suite, but are not 3d relevant.
So the restriction in place that prevents downgrading to Yosemite on the iMac's actual hardware (which I believe is something to do with the GPU) is irrelevant when it's powering a VM?
It is more likely something to do with the CPU and while it is not completely irrelevant, but this is something that a VM can shield off.
If your host has a newer CPU as the guest OS supports then you can actually show the guest OS an older model of the processor.
This is called CPU masking, by masking off the newer features that your guest OS doesn't support, it does not end up getting confused.
Not too long ago there was an issue like that where Oracle refused to run on newer macbooks and it was fixed by CPU masking.
I would expect that Fusion does the CPU masking on your behalf, not that you would have to edit it by hand and if not you can always report it here and we try to help out.