The big one here is IMO the change in respawn systems, from group respawn (the thing they custom-made for Season 7) to a more standard wave respawn system:

So those things were great, but the biggest problem with Group Respawn was how dying first often led to getting “buried” by a long sequence of teammates dying in a row, making your respawn time a lot longer. This meant their respawn times were generally shorter, but it also meant yours was longer. Wave Respawn fixes that by ensuring your respawn time will always be shorter than the default if another hero was already dead.

Though the statistic with group respawn is truly interesting, about how it made a much bigger difference in particular in the highest ranks. Which is curious, as you'd also assume that in those ranks, people would have known to stay back and wait for their respawning teammates, which in a way is all group respawn ever did, forcing that.