Currently Barbarians lose their level 7 second furore if they die. As they're quite expensive and it's some times hard to come across the one you want, I suggest that, like rogue perks, barbarians keep their learned furores no matter their level.
People might level up then die down in order to build better and have two furores, but if that became an issue, the level 7 furore could be made pure only.
AFAIK you do keep it. You just need to hit level 7 again for it to activate. So you don't lose it, its just inactive if you don't meet the level requirement.
It is true, you keep it and just need to hit 7 again.