Do you have proof it wasn’t? Unless you have proof to the contrary then it’s still a reason as to why it hasn’t come back.
I was surprised that we haven’t had a reoccurring charity event because as stated before, WoW does. And if I recall (or at least during the time I’ve played) you could get the previous year’s pet for a donation.
There also:
So unless we start seeing hints at new possible charity events I don’t think Pink Mercy will becoming back as just a purchasable skin.