In Overwatch, Genji has been enjoying a relative heyday, with everyone around him being nerfed. D.Va and Roadhog were two of his biggest threats, one a mobile, indestructible killing machine and the other a one-man wrecking ball who could pick you out of the sky. Winston was never really a problem, so with D.Va and Roadhog's nerf, Genji has began his life on the top.
But it was not to be. Winston's huge shield buff, in essence cutting the cooldown on his barrier in half, makes him a lot more survivable. Since Winston doesn't have to aim, Genji's mobility is for naught, and any other buffs to Winston is always bad news for Genji mains.
It's not that Genji is directly affected by barriers, or Winston's barrier. He's not. But, before, Genji could just wait until Winston was shredded by his teammates, as his survivability wasn't worth a damn. Now, Winston can plant firmly in the team's line, protecting healers with his massive body and barrier, and, when Genji comes around to pick off the supports, he can't count on his teammates to take care of the big ape in the mean time.
Similarly, Winston is so good at diving in, that even a patient, defensive Genji isn't safe. Winston can easily jump in, drop a barrier, do massive damage to Genji without aiming, and jump out before anyone was the wiser, dropping down another barrier for good measure.
This is what we like to call a ghost nerf. Like, when Ana's damage got nerfed recently, since she was once of the main counters to Pharah, Pharah got a ghost buff.
We won't know exactly how this affects pickrates, but I'd expect to see fewer annoying Genji's roaming around if you have a halfway-decent Winston on your team, especially since Genji mains are already complaining en masse on the Forums and the Sub Reddit. Just like immediately post-Hook 2.0 Roadhog, all good things must come to an end.