Is it safe to discharge a water softener into a saltwater swimming pool?
It will depend what's in your water, among other things - the last place I used a softener, the clear water iron made the softener discharge a very rusty orange. Not exactly appealing for a pool.

If you add salt to a salt-water pool that is presumably losing water primarily via evaporation, it will become hyper-saline over time. I'm not sure how much this would be regarded as a problem in a salt-water pool - perhaps you want it to be like the dead sea - very buoyant.

Even if you don't have iron turning the discharge water rusty, the calcium removed by the softener will accumulate in the pool, and may cause some water quality problems there.

On the whole it seems more likely than not to be a bad idea, but if you are determined to go ahead with it, I'd suggest that you start by catching a bucket of the discharge water and let it sit for a few days, to see if that offers you any insight on what you get with your particular water.