This has been a request seen on the forums many times, and I promise you Blizzard is aware of the interest from some members of the community about skins like Pink Mercy, past Overwatch League All-Stars and MVP skins, etc. Nevertheless, there are no official plans to bring this skin back and will remain as a Legacy listed item (Legacy items have no intention to be returned to distribution in the game). Could that change? Yes, but extremely unlikely that it will.
My advice is to look forward to the future and make sure to no miss out on cool exclusive skins in future promotions. I myself try to help with this by providing key information about streaming promotions here on the forums and I also run a handy Twitter feed called @dropswatch which alerts followers to new limited-time items either earned in-game or through streaming promotions.