Tracer and Zed were the turning points for their respective games.

With those games of course being Heroes of the Storm and League of Legends. My personal opinion is that Heroes of the Storm went sour fast after the introduction of Overwatch Heroes (nothing against Overwatch itself or it being represented in HotS, but because of HOW Blizzard chose to represent it), and League of Legends started going downhill with the release of Zed. To League's credit though, it's been a slow burn. Blizzard basically ruined HotS overnight, which is embarrassing and tragic.

What do Tracer and Zed have in common, and why were they the start of something bad? In a nutshell, the marriage of untaxed mobility and "alternative" resource management, slim-to-nonexistent vulnerability windows, with any existing windows being mitigated by the aforementioned mobility. Also, critically, many of the older characters are simply incapable of dealing with these new ones, and it's not because they are "outdated." It's because they got statchecked by their creators who have shifted their development philosophy away from properly budgeting power. Tracer and Zed aren't the worst offenders when it comes to invalidating the rest of the cast (Genji and Yasuo come to mind), but they were the start of this decline for their respective games.