Back in 2017, from January 24th to October 2nd, Pokemon Bank gave players Mewnium Z if they had a subscription.
What I don't understand is why? Going by Serebii's event page, the US hasn't gotten a Mew event since X/Y/OR/AS had gotten one since February 2016. The only Mew events to run in 2017 were a Japanese one and a Korean world championship event.
So why give this out in an event, without a Mew to tie along with it? It seems to me like most players wouldn't be able to get one easily.
Especially considering the GTS no longer permits players to trade mythical Pokemon.
It feels even more worthless to me considering that Ultra Sun and Ultra Moon gave you the ability to get this, if you had a Mew. Which seems to make more sense to give it out in that manner.