Using the sauté function on the Instant Pot, sauté the garlic in olive oil and 2 tablespoons of the butter for about 2 minutes, until softened and starting to barely brown.
Add the wine and cook for 3-4 minutes.
Add the black pepper, red pepper flakes, salt and broth and stir to get any stuck pieces off the bottom.
Add the linguine to the pot, making sure it's fully submerged in the water.
Place the frozen shrimp on top and seal the lid. Pressure cook on high for 3 minutes.
Naturally release pressure for 5 minutes, then use the quick release to release any remaining pressure. (Reduce the time slightly for smaller shrimp to avoid overcooking them).
Stir in the remaining 2 tablespoons of butter, lemon juice and parsley. Adjust salt to taste.
If there is still liquid in the pot, simmer it on the sauté setting for 2-3 minutes until mostly absorbed and pasta is cooked through.
Serve hot with lemon slices (optional).