More of a commet than anything
I was stunned when I read in a PDF that there were no OSX Mojave drivers for the Phaser 6125. I do not use OSX much other than playing around, but I took a chance and installed the package for 10.10 & 10.11 and i works great as ever.
If this printer driver issue is holding you back from your Mojave update , you can be assured that I had success and have printed several tests with no issues.
Drivers are listed for OS they were developed for, or at the least tested on.
When a company stops developing drivers and software, they simply stop testing on new OS's. It doesn't mean they stop working. In the case of Windows 10 and OS 10.14, driver signing/signature enforcement sometimes will need to be disabled in the OS to allow their install.
The biggest battle by far recently is Apple's Library validation, which makes it impossible for print manufacturers to have extended features within Apple's apps (Safari,Pages,Preview). So if you notice that you can't make booklets or use Accounting systems when printing (Easiest check is you will see Printer Features instead of the manufacturers Xerox Features/ or Epson/Toshiba/Konica/Canon Features)
If you get stuck in that situation, swap Safari for Firefox/Chrome, swap Preview with Adobe Reader etc.