Events and In-Game Content

From all the information I could gather, the Mythical Pecha Berry Mystery Gift event seems to have no end date. Is that because it unlocks in-game content? If so, why weren't the four Mystery Gift events for Brilliant Diamond and Shining Pearl given the same treatment? I think it's unfair that people like me who got the games late are permanently locked out of said in-game content. Can you please redistribute all four of them and not give them end dates this time to rectify this?