Now that the new Mac mini 2018 are in users' hands, the question has been answered. Yes you can update the RAM modules in the Mac mini 2018, but it is way more work and complicated than doing so on my Mac mini late-2012. Here is a link to a
Mac Rumors article that discuses it.
Perhaps many of the people in forums like this one can accomplish this process, but I still think that the average Mac mini 2018 owner would not have the skill level nor tools to attempt it. As the article states, it requires an enthusiast level of expertise. IMO, an enthusiast level of expertise falls somewhere between the novice and professional levels, and certainly way closer to the professional level. This is NOT a simple task like it was with my Mac mini late-2012 where you open a trap door and simply unplug the factory RAM modules and plug-in the 3rd party ones. The Mac mini 2018 requires one to detach the bottom plate with a tool, unscrew and remove the antenna plate, unscrew and remove the fan assembly, unscrew and remove the main board, unscrew and remove the RAM cage, remove the rubber stabilizers and press the spring clips, then replace the RAM modules, then reverse these steps to put everything back where it goes.
If you are an enthusiast, then go for it. If you are not, either find a friend who is or pay more to Apple to get more RAM installed.