I note that High Sierra is no longer available for download (from app store or in purchases list), and to rub salt in to the wound none of the methods to create a High Sierra USB installer (or Sierra) appear to work any more, I have previously downloaded installers as I suspected we’d get to this point. I’ve tested on 3 different Macs running 3 different versions of macOS/OS X - none of Diskmaker X, recommended Apple command line and Install Disk Maker from macdaddy.io work on and of Mojave, High Sierra or El Capitan.
I’m not a big fan of Mojave but now appear to be stuck with it - I could try an Internet recovery to the original version of the OS that came with the machine (Lion) then try to update to High Sierra but given the above issues I think the chances of this working are zero.
I am SERIOUSLY pissed off about this it is utterly pathetic that I can’t install a previous version of the OS