Not only as askunk mentioned is this unlikely to be the Xeon version... But if you assume after all this time that Apple will release the moment new chips are available... well...
As mentioned immediately above the code name 'Cascade Lake' primarily covers Xeon options so the issue is far more so where specifically 'Xeon W' falls.
Xeon SP probably will ship in Q4 2018. One of the
primary features of the Cascade Lake update is hardware fixes for Spectre and Meltdown. (
https://en.wikichip.org/wiki/intel/microarchitectures/cascade_lake#Key_changes_from_Skylake ) . Very large virtualization service vendors that need max high security VM servers will not be put on the "last to get" list for this update. Intel would have to certifiably looney toons to do that. That is primarily who is going to pay top dollar as fast as possible for this stuff.
There are no big core count increases. No huge clock speed increases. There is a narrow "AI" targeted expansion to the AVX-512 instruction set and some memory controller updates ( speed bump, density bump , and Optane as DIMM bump). That's is about it. It would make sense for Apple to wait on a Mac Pro update so they never have to deal with the security kludges with the new model. There is very little gap between the current Xeon W chips and the next one physically or electrically. Apple could design mule the who system with the current Intel W (a derivative of their already existing iMac Pro logic board) and just be vulnerable to Spectre/Meltdown. The super duper memory options are probably not relevant to the Mac Pro (or iMac Pro). The one AVX-512 opcode tweak really can't be mainlined into macOS X until that feature if rolled out more broadly in 2019-2020. At this point Intel has engineering samples of SP and W variants already out. So a firmware update of mule board and would have all of second half of 2018 to squash out bugs and design tweaking. There would no impediment to finishing engineering verification test by end of 2018.
So SP will at least ship to the "big 5" Cloud service vendors in Q4 2018. (they always get stuff 'early' ) Whether "normal" folks can get them in 2018 or not might be questionable. They should be more widely available in early 2019. The Core-i9 stuff..... may go in 2018. ( Again Intel isn't going to cry tears if there is limited availability and spot market Extreme Core i processors prices are kept high for an extended amount of time.) So, there is pretty good chance that Xeon W will slide to early 2019 ( even more so if Core i9 goes in Q4 2018 with limited availability. )
The notion that Apple doesn't have the hardware components available to do a Mac Pro in first half in 2019 is ludicrous. The
only reason a new Mac Pro would slide to second half of 2019 is sloth development investment (and activity) by Apple over the last 2-3 years. Period. There is
no 3rd party slowing them down.
Over last year, Apple could have targeted components projected to be available by end of Q4 2018 and picked a first half 2019 target window. That would have been safe. It would be beyond foolish for Apple to have selected 2019 parts back in 2017. That would be just mind boggling dumb for a product that is
grossly behind schedule by the customers perspective.
