So if I were to attempt to flash it via fastboot, it would give me an error and ask for confirmation before proceeding, correct?
And if I see said error, stay away.
If you flash a stock firmware, you'll see the error, but it won't ask you for confirmation. It'll just display 'security version downgrade' if your bootloader is actually newer than the firmware you're flashing. Thus, fastboot should block you from downgrading the bootloader and GPT, so you'd likely still have the Oreo bootloader on your device (unless something's changed with Motorola's enforcement of bootloader versions).
The only time you may be okay is if the subsequent OTA is the same as the newest firmware that you had on your device. E.g. if you applied the soak test OTA (OPS28.49-2), then downgraded to the April 2018 firmware, if the next OTA you use is OPS28.49-2 or a newer Oreo OTA (as you have an Oreo bootloader), then you should be okay. If you however get a
Nougat OTA (like the June 2018 security patch), I'd honestly stay away unless you want to risk it. Again, I do not know if the June 2018 7.1.1 would brick your device, but you've got no way of recovering your device besides paying for an expensive motherboard replacement. If you're on the soak test, try to stay on the soak test please.