Mercy received a significant overhaul in the last major Overwatch balance patch, completely changing how her Resurrect ability works. It seems the new Mercy was a bit overpowered—she's been incredibly prevalent since the changes went live—and now Blizzard is testing a nerf to her new kit.
The latest iteration of the Overwatch PTR changes Mercy's new ultimate, Valkyrie, making it no longer reset or reduce the cooldown of Resurrect. Before the change, Resurrect's 30-second cooldown reset upon activation of Valkyrie, and was reduced to 10 seconds for the duration it was active. This allowed Mercy to rez several teammates in rapid succession—in addition to the massive healing, damage, and mobility boost that Valkyrie provides her personally. On the PTR, Resurrect now retains its 30-second cooldown while Valkyrie is active, though the ult does double Resurrect's range.
It's a small change, but hopefully one that will allow Mercy to still feel powerful and fun to play, but not feel like she's a must-pick in all situations. After all, poor Ana was feeling a bit left behind.
As always, balance changes on the Overwatch PTR are subject to change.