Anyway...
With 2 forums (early/late) I'd consider the minimum MacOS as a consideration, rather than get too strict on features. Anything that natively supports 10.7.5 or lower is Early; anything 10.8 or above is Late. That gives us a split around 2012, and essentially splits Intel around the final Steve Jobs / Bertrand Serlet OS, as well as being the effective transition from iOS-ified hardware supporting modern features.
After beginning to flesh this out last day, I considered both a) physical network capabilities, out-of-box (i.e., not needing dongles) and realized this is an imperfect metric, since that remains fairly consistent (on the Ethernet side) whilst the standards for the wifi/BT side have nothing to do with Apple (and can be fudged, at least for certain models, by swapping in a module from a later Mac); and b) OSes (which, aside from the self-contained restore partition scheme premiering with Lion, was also kind of a non-starter, in that OCLP and prior patchers have bridged a lot of those waters with Apple-unsupported models; dosdude1 is such a lovely, amazing disruptor, isn’t he?).
[Also, Serlet left after Snow Leopard and some time before Lion was finalized, leaving it to Craig Federighi, who still runs that hidden-restore partition/yearly major version release/free-with-telemetry show that is the modern macOS.]
So I returned to, specifically, what Apple included, of their own volition, in their own hardware, with the only consideration to operating systems given to the fuding of being able to get Snow Leopard running on Ivy Bridge architecture (and, probably, Sandy Bridge-E, if there was a way to flash the MP5,1 firmware to accept post-Westmere Xeons).
To simplify even further, so that absolutely no-one would be confused, it should just be by year: 2006-2012, and 2013+. They're called "Classic" Mac Pro for a reason!
They’re called “classic” because they use the same case, as borrowed from the G5. Internally, the trash can wasn’t at all a transplanting of the core MP5,1 components, but rather, a clean-sheet re-think (which was, well, a mixed bag) even if they shoved it all into the same “cheese-grater” case we all know and love.
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