How to transfer printer drivers from one mac to another?
I went through this yesterday and I found a solution -- at least a solution that worked for me.  

I have an old Xerox WorkCenter printer for which Xerox no longer produces OS drivers ... but the printer works great.  After loads of experimentation, here's what I discovered:


Folders such as /Library/Printers/... seems to be a repository of drivers but these are not drivers that are in use ... they are simply drivers that could be selected to support a printer that needs such a driver.
When you install a printer, it will search the folder for matching drivers.  
The driver you select will be copied to /etc/cups/ppd and renamed based on the name you gave your printer.  (Note that the top level /etc folder is a symbolic link to /private/etc these days)


Example:

My Xerox WorkCenter C2424 was working on another mac and the driver was /etc/cups/ppd/WorkCenter_C2424_93_93_1b_.ppd   This name was previously auto-generated by the other mac based on the final octets of the printer's ethernet address.

I copied the above file to the new mac but put it in the /Library/Printers/PPDs/Resources/Contents folder and renamed it back to just Xerox_WorkCenter_C2424.ppd.

I then used the "System Preferences" -> "Printers &amp; Scanners" to install the printer on the new mac.  Except rather than letting it auto-select the driver, I chose the "Other..." option which opens a file navigation window and navigated to the /Libary/Printers/PPDs/Contents/Resources folder and selected the driver I copied and renamed from the other mac.

This caused the mac to copy that PPD to the /etc/cups/ppd folder on the new mac and to rename the copy based on the name given in the printer list (you can give it any name you want).

I tested the printer to make sure all features worked and verified this did indeed get the driver to the new mac (the new mac is running Mojave).

I'm not sure if I'll get away with this trick whenever it comes time to upgrade to Catalina ... since Catalina seems to have strict rules on not supporting 32-bit code and these very old drivers are probably not 64-bit drivers.