It's safe for goats, as far as I've heard. Keep in mind, though, that what works in horses won't always work in goats, and vice versa, becuse of the radical difference in digestion processes.
I have about 50 lbs of ground flax seed in sealed packets. It was human-use flax that "expired" but is perfectly good as goat feed. I use it to encourage an ill goat to eat and feed it to baby kids to get them an early start on grain. For some reason, kids will start eating ground flax much earlier than they will try ground corn or oats.