This is the first I've heard of NTRU. Has it been proven immune to quantum computational attacks, or has an efficient attack algorithm just not been found yet?

The former would be world-shaking, so that's probably not the case unless I've been living under a rock. The latter just means that it might be safe, but it might be broken at any time if somebody comes up with the right algorithm. If you're making long-term information infrastructure plans that's something you have to account for.