For some time now, our customers have relied on our EFI cards to allow File Vault to work. Apple pain ted themselves into a corner on this, disabling 'non-metal' cards in Mojave. Since they never sold a cMP with a Metal card inside and their deal-with-the-devil (AMD) cards do n't have EFI, they couldn't REQUIRE an .EFI card for File vault for Mojave.
Instead of rewarding the folks who kept their machines current, they decided to punish these people instead. they make it look like you just have to turn off FV to install Mojave. But they intend for you never to use it again. All so they can avoid admitting they've been petty, vindictive jerks about Nvidia.
Anyhow, it seems that there is a workaround that involves dosdude and his patcher. I haven't tried it myself yet. (I've been busy meeting demand for GT630/640 metal cards, oddly enough, which don't require Web Driver and work OOTB)
So as the driver gets released and more people upgrade, please post your stories here. Is FV working for you?