One thing that does influence apple support (that needs to be considered) is support from the OEM for the hardware contained in it.
If the CPU in the iMac Pro has gone end of support from intel, then Apple have no way they can continue to support it for say, unforeseen newly discovered security issues that require CPU microcode update.
With A and M series it is entirely up to Apple, but for machines that contain third party components (CPU/GPU/Network adapter or wifi adapter chips, etc.) - Apple will not officially support them if the OEM has stopped support, as they themselves need upstream OEM support to update it.
So yeah, AMD no longer support VEGA discrete GPUs - so anything containing a Vega discrete GPU in either PCs or Macs will no longer have support. That rules out the iMac Pro; Apple can't do new features on metal, etc. that may require driver programming support for Vega as AMD has EOLed it.
The Mac Pro 7,1 is also going end of support because its CPU is out of support (ditto for iMac Pro as it is older).
e.g.,
Intel® Xeon® W-3223 Processor (16.5M Cache, 3.50 GHz) quick reference with specifications, features, and technologies.
www.intel.com
end of intel product support for the 7,1 Mac Pro CPU was June 30 last year.
Which means any catastrophic security bug discovered in it that needs microcode updates will not be provided by intel - and Apple can't create those themselves.
"Support" from Apple in terms of software, etc. means they will generally patch or otherwise mitigate security problems. In the above two cases for example, they simply can not guarantee that.
It remains to be seen how Apple will treat M series. The initial M1 processors are not yet 7 years old which has generally been the beginning of the end for software updates in the past (with a few exceptions).
They've been pretty good with A series chips lately (iPhone X still getting updates/support!). The ability for Apple to provide ongoing software support is totally up to them.
Some of these (older, non apple) parts will continue to run on Windows, as Microsoft usually don't actively block hardware (though this is starting), the driver support is left to the OEM; if your old Vega GPU works with Windows 12 for example (e.g., maybe using a legacy Windows 11 driver for example), all good. If it doesn't, you need to cry to AMD for a driver, it's not Microsoft's problem.