Nothing proved it wasn’t, the devs never said anything that it wasn’t, players never said it was OP, people just didn’t like it. In this games history, only two heroes have been reworked because players didn’t like their play style, not because they were OP. Mercy and Hanzo. And since their reworks both of these heroes have dictated the meta.
Every other hero who has been reworked was reworked because they were underperforming in what they were initially intended to do. Bastion, Sombra, Symmetra, Torb soon enough. Those heroes under perform and are being changed because of it.
Mercy needs a revert back to season 6 with LOS added to ult and get rid of invulnerability as all it did was enable hiding and going for a mass 5 man rez because, if no one remembers, before it was added it was a death sentence trying to get one. With LOS and no more cheap escape plan you would be punished hard for even attempting it.