It is true that you are only allowed to craft these powerful Fish Oil elixirs during the Arcane Anglers Fishing Derby event. The event was a success, so it will definitely return! For this reason, I thought it was fair to let you keep the recipes (instead of removing them at the end of the event). When the event comes back the rules will be the same: you'll be able to craft during the Derby and during the event cool-down, but not afterwards. And, if the recipes remain extremely rare, you'll be able to keep them like you can now.

So TLDR: no Fish Oil crafting until the event comes back but you can keep / trade the recipes.