In a topic on the Blizzard forums noting that Tracer hadn’t been touched in two years, a fan inquired as to why the speedster is currently considered one of the strongest characters in the game. Game director Jeff Kaplan popped in to share some valuable insight on the ever evolving meta, balance concerns, and player perceptions. He writes:
The perception of balance is often more powerful than balance itself.
All heroes are affected by changes to other heroes. Even if nothing changed, players would adapt new strategies and tactics that would cause the balance of the game to shift.
If you look back to a few months ago, a lot of people were demanding Reinhardt nerfs because he was a "must pick" hero in the meta. At that time, Winston was literally the least played hero. Fast forward to today, and we're seeing Winston as the dominant tank choice in the tournament scene (along with D.Va) and Reinhardt's play time has dropped significantly.
Short of the adjustment to Earthshatter (not intended as a huge balance nerf but rather more of a "that doesn't seem right" fix), we did not touch Reinhardt's balance. But changes to other heroes and changes to the "accepted" strategy of how the game should be played at a meta level has changed which tanks are perceived to be "right" or "wrong" to play.
Overwatch’s professional meta has shifted into a few distinct phases, including the three tank meta and the popular Beyblade combo, where an Ana would nanoboost a Reaper, who would then ult. These combos fell out of use as heroes were buffed or nerfed, or as teams figured out combos.
Ana is an excellent example of a hero whose strength was misunderstood; after her release, she was widely considered one of the worst supports. After a few small tweaks, she then rose to the top of the support meta. Blizzard then had to shave power off of her, including the speed boost off her ultimate, and the meta shifted around her.
Kaplan comments on how hero changes can have a butterfly effect that change the meta around them:
You can make a similar argument about Zenyatta's increased play time. Zen's new prevalence is due to adjustments to other heroes -- not Zen himself -- as well as a shifting meta.
So Tracer's balance -- beyond the obvious tuning knobs that we have -- is greatly affected by the balance of other heroes as well as the "perception" of how good she is at any one time.
It's always fun to study how people innovate to "break" a meta. In the NHL in the 90's teams developed an overly defensive strategy (think 3 tank meta) called "The Trap" which was later evolved into "the Left Wing Lock". Teams like New Jersey and Detroit won championships using these methods. Eventually, the NHL had to make rule changes to "change the meta".
Based on this, it seems unlikely that Tracer will be getting nerfs or buffs in the upcoming patch. However, the changes to other heroes—including Reaper, Roadhog and McCree—may push her out of the top spot.