Now that macOS Catalina has been officially released, it might be a good idea to also download a copy of the macOS Mojave installer and archive it somewhere on an external drive. You’ll never know when you need it.
The good news is that Apple didn’t remove it from the Mac App Store. The not-so-good news is: good luck finding it.
Fortunatelly there’s this KBase article that has a direct link to macOS Mojave.