With the addition of Mercy's new ultimate ability Valkyrie, it was expected that we would receive some new voicelines when she activates this. Mercy was still using her previous voice line "Heroes never die!" until an update hit the ptr today. Her new voice line from an allies perspective when activating is interesting. Mercy says "I will watch over you!" instead of "Heroes never die".
Mercy's Ult From Allies Perspective | Source: JustHarmony (Reddit)
It is a shame to see Mercy's famous "Heroes never die!" voice line go, but we are sure that this will still be used when activating resurrect. She also has a new voice line when activating Valkyrie from an enemy's perspective. She says "I will watch over you" again but this time it is obviously in the country that she comes from which is Switzerland.