You’ll hit an OOM condition and crash.
However - this shouldn’t really be happening anyway because a large portion of RAM is supposed to be used for ARC cache. If you start hammering the memory, ARC will resize dynamically as needed - even in extreme circumstances.
See: ARC Memory Allocation on Dragonfish TrueNAS 24 and Other Issues - #11 by essinghigh
In any case, you don’t need to swapoff. You can just disable lru_gen.
echo n >/sys/kernel/mm/lru_gen/enabled
This is the fix that iX will be using, they will be defaulting new installs to not have swap as well, but disabling lru_gen is the actual fix for the unnecessary swappage seen with dragonfish .0