I bought the mid-2018 version so as to be guaranteed to be able to downgrade to macOS High Sierra, since the mid-2018 version originally shipped with that. Even so, it came pre-installed with Mojave and I jumped through many hoops trying to downgrade it while it repeatedly refused. I eventually was successful by completely wiping the internal SSD, reinitializing the file system as HFS+, and restoring a Time Machine backup of my iMac onto it. Now it is a clone of my iMac running High Sierra, both running off of HFS+ SSDs, and I am very happy with it.