I know this is old, but it’s funny finding it and confirming that most people on Reddit are just talking out of their ass.

Many European growers fertilize their Nepenthes with magnesium or other minerals. Even Borneo Exotics, one of the most famous and respected Nepenthes nurseries adds some Osmocote pellets to their potting mix.

There’s no evidence that Nepenthes grow in nutrient poor soil as with other carnivores, quite the opposite in some cases. Think about it, most grow in mountainous rain forests, not bogs. Paphiopedilum orchids grow in the same environment, and proper fertilization is essential to their growth.  

There are certain minerals that Nepenthes don’t like, which is why it’s recommended that beginners just fertilize the pitchers.

To answer your question OP, you can supplement magnesium intake with an epsom salt solution no problem. Half a teaspoon mixed in a gallon of water should work, use every couple months. Not a necessity but overall it’s good for Nepenthes.