I managed to set get_unlock_ability back to 1 again (with an unlocked bootloader, don’t get your hopes up) by using Magisks resetprop…
adb shell su -c 'resetprop ro.boot.flash.locked 1'
…and switching the OEM toggle in developer settings back to on.
Magisk for the rescue again 
We should advise people to check fastboot flashing get_unlock_ability before locking the bootloader.
It’s a bit of a hassle having to set up Magisk just to reset that flag, but beats bricking the phone. Maybe there’s another way to fix this 