As written on MacRumors and other places Macs using 3TB Fusion Drives for the macOS and Boot Camp installations can't be upgraded to Mojave currently without losing the Boot Camp partition.
To fix this problem, Apple says that customers with the 2012 27-inch iMac with 3TB hard drive will need to completely remove the Boot Camp partition using Boot Camp Assistant before macOS Mojave can be installed.
After upgrading to macOS Mojave, Boot Camp will not be able to be used to install Windows on these machines. No other iMac models appear to be affected by this issue.
What is the technical reason for this?
PS: See also Apple knowledgebase document (which mentions only Late-2012 27-inch iMacs with 3TB fusion drives.)
Best Answer
The technical reason is that the fusion drive is an Apple engineered disk spanning software layer and Apple didn't implement / ship this for fusion drives on top of APFS. Someone has to write the actual code that runs behind that Boot Camp assistant program and the code was written to not start on that specific hardware.
Now - that decision was probably a business decision. We could speculate that it required tradeoffs or more engineering talent or failed some performance or stability test so they removed that capability technically during the planning of the OS before it was publicly released to beta in June 2018 and shipped in September 2018.
Boot Camp is three things:
1) Drivers to make Windows see the Mac hardware as legitimate
2) Scripts to repartition the drive.
3) Documentation and articles like the one you references to educate people how to use the software and what to expect.
Apple could have decided to leave these few Macs without the entirely new APFS and coded other exceptions, but I'm not at all surprised they just said - no bootcamp on some small subset of iMac that didn't ship with SSD as the main storage.