Where did you get the idea that prawns contain thiaminase and shrimp do no not? For that matter I was of the understanding that shrimp and prawn are virtually synomonous, the different words referring to size, but different cultures use them differently. British (and commonwealth) english uses "shrimp" for small and "prawn" for large. American english uses "shrimp" as a more catchall.
https://www.practicalfishkeeping.co...ed-to-know-about-predator-fish-and-thiaminase says that thiaminase is found in cyprinid fish, mussels, and
crustaceans
Both shrimp and prawns (which are common name classifications not scientific; both are decapods as are crabs, lobsters and crawfish) are crustaceans. In fact, a fish fed on nothing but brine shrimp and/or daphnia might also have problems.
Thiaminase will probably be destroyed by cooking, but it also probably destroyed all the thiamin in the food before cooking.
Regardless, shrimp and prawns should not be the main diet. I think the main issue about thiaminase is that feeder goldfish are a really bad choice of live food.