Yes frozen onions are still safe to eat.
You can freeze onions and then thaw them out and use them in soups, stews, Chili etc and the onions will be just fine and safe to eat.
For example onion rings are onions that are cut up and then those onion rings are frozen until they are sent to the store and they are frozen at the store and then when you bring them home they are frozen until they are ready to be cooked.
The same is true with places where you order onion rings as those will be frozen as well.
There's no way for onions to become poisonous just because they've been frozen.
I've never gotten sick from eating frozen onions once I thawed them out.