12 Nov - Big Sur 11.0 released
26 Apr - Big Sur 11.3 released with RX 6800 XT/RX 6900 XT driver support
9 Jun - AMD released Radeon Pro W6800/W6600
3 Aug - Apple released 6800/6900 MPX
It doesn't mean CPU will follow similar trajectory but you could guess.
8 Jun - 'Rumor/leak/discovery' spread that Monterey will support Ice Lake
29 Jul - Intel released Ice Lake Xeon W-3300 series
+ two months
Oct/Nov - Ice Lake Mac Pro with the release of Monterey 12.0?
More likely Nov/Dec 2021. Not sure why Apple would want to ship a hard core pro machine with a x.0 OS release.
Apple released an "Ice Lake" MBP 13" 4-port model back in mid-2020.
en.wikipedia.org
I highly doubt Apple is going to put much effort at all into the AI/ML opcode additions in the Xeon W3300 series that the Core iX 10xxxx don't have. Likewise for the AVX-512 differences ( Apple is completely ignoring AVX in Rosetta so they aren't putting effort into writing more stuff that can't be translated. )
macOS has had to deal with the baseline "Ice Lake " ( Gen 10) microarchitecture for over a year at this point. This "new" CPU shouldn't be as big of a hurdle to jump over as the GPU graphics were ( RDNA -> RDNA2 which actually is a baseline architectural change ).
Right now though is a bad time to release a new Mac Pro. Apple has all of the WWDC betas in flight. Bug fixes on the current macOS/iOS/iPadOS in flight ... Yet another system at the moment likely won't getting the proper attention to detail that the Mac Pro deserves. ( few want to deploy a flaky OS into a critical production workflow. )
If Apple is sane they shouldn't want the Mac Pro to ship with macOS 12.0 ; extremely likely that 12.0 is going to ship with a short list of known substantive glitches. ( Apple's track record of last 3-4 Fall releases speaks for itself. A flurry of "oh yeah that's is broke" fixes over the first several months. )
Catch 22 for Apple when they drop the volume of their Intel CPU buys off the table then they probably drop out of being a top-tier customer for Intel with "front of the line" access to limited volume products. The fewer Intel CPUs that Apple buys , the more to the "back of the line" they are likely going to get shifted. Intel is going to sell a decent volume to Apple over time, but they aren't going to brush off Boxx, Lenovo, Dell, HP if they want to place orders for W-3300 series. I doubt Apple gets volume shipments for these chips before late October or so. They can't make completed systems if don't have CPUs to stuff in them. Then they have to ramp the factory for 3-4 weeks so have enough inventory for the intial demand bubble. So likely well into November on that front also.
The timing also fits a 2-year gap since MacPro7,1 release. Leave sufficient gap for the new half-size Mac Pro with Apple chips in the first-half of 2022.
Then Apple could declare in WWDC2022 they've completed the transition in 'two years' as envisioned by Supreme Leader Tim Cook the Great. The most powerful x86 Mac ever released in the next decade lives on together happily with its younger siblings.
+ another year (i.e. two years after the Ice Lake Mac Pro refresh)
I suspect that "completed transition" is going to be a bit of "move the goalpost". Complete the transition is going to be have a new M-series Mac in each Mac product category. It probably won't be that they have completely turned off all x86_64 system sales in 2 years. Depending upon how Apple addresses the mulitple GPU solution space this update Mac Pro probably would continue on for years if the M-series macOS can't handle anything other than one GPU for the same multiple years.
The final replacement may (or may never) arrive with Apple chips. That's around 2023. Fast forward. By around 2025+ x86 MacOS support dropped publicly.
That is probably more on the '+' side than 2025. Apple still has the Intel iMac 27" , 4-port MBP 13" , MBP 16" , upper Mini for active sale. That "de-support" clock for macOS x86 isn't going to start until apple discontinues those systems completely. The iMac seems to be sliding into 2020.
And while the M1 (1st gen) systems are faster than the last iteration of the x86_64 Macs , most of those x86_64 Macs are "fast enough" for their users for many, many years into the future. There is lots of inertia there; for more than what PPC had back in 2005-2006.
If the Mac Pro 2021-2022 is out there 'dangling' as the last hold out they make start the clock before it hits withdrawn from sale date. However, if Apple stops with the "half sized" Mac Pro replacement , they probably won't pull the plug early.
Personally I hope by 2025 high-end AMD/Intel processors outperform Apple chips by a significant margin to keep Apple checked. Perhaps prolong MacOS support on x86.
IMHO, highly doubtful Apple is going to chase them in the "core count" wars. Apple will probably try to have a high overlap in tracking their workstation offerings. The more single user on more mainstream "pro" A/V/Media stuff focused than in top end HPC or top end AI/ML.
Apple is probably going to sit in the "do best you can under 64 cores " limit. Farm more specialized workloads they are targeting out to specialized cores ( NPU , GPU , AMX , etc. ) . That isn't going to keep Apple "checked". Apple is going to spend more effort targeting the subset that want to hold onto (and not try to do everything for everybody at the top end. )
2025 AMD/Intel will probably be more competitive with M-series of the same timeframe, but very doubtful that is going to convince Apple to continue on the path of marrying at T-series to an x86 processor as an alternative. Apple's lead on process fab access will probably sag over time ( AMD isn't as "poor" anymore and Intel also not poor and cleans their fab house up a bit ( or a lot ) . ). Apple doesn't exclusively own the tech, they just write big checks. There is no huge barrier to entry to writing a big check if have the money in the bank.