Since the slug is dying of dehydration when pouring salt on the slug they will have pain when they die of dehydration.
So yes slugs do feel pain when you put salt on them and they die.
The salt dehydrates the slug which is a painful way for them to die.
When you pour salt on the slug it rapidly dehydrates the slug and can also knock their eyes out so they do feel pain when you dehydrate the slug with salt.
The reason that salt kills a slug is it causes the slug to dehydrate and when they are completely dehydrated the slug dies.
The salt dehydrates the slug by mixing with the water in the mucus that the slug secretes to help the slug move around.
When that salt mixes with the mucus it creates a salt water solution and then dries out the slug which dehydrates them.
When you do pour salt on the slug then the slug will lose so much water that it dehydrates, dies, and winds up looking pretty shriveled.
Using salt on a slug or slugs is a natural remedy that kills slugs and gets rid of slugs pretty fast and safely without use of any chemicals.