This nerf came out of the left field, and their vague explanation located on the 6.4 PvP Guide doesn't hold water:

Silent Nocturne is an incredibly effective tool to deal burst damage to enemies in conjunction with attacks from teammates. Because of the difficulty in countering this ability, however, we believe it required significant adjustments. We've reduced its range to both force tougher decisions in choosing targets, and increase risk in its execution. This should also give opponents a better chance at potentially dealing with the attack. To compensate for this, the potency of Apex Arrow and Repelling Shot has been increased, improving bard's area of effect and close-range damage potential.
Many jobs have incredibly effective tools to deal burst damage. Also, other jobs have difficult abilities to counter. We don't see any of those getting significant adjustments. There were other ways to tone down Silent Nocturne without completely gutting the ability. The range nerf is the most egregious part of this change. Silence effect too long? Fine, bring it in line with RDM's 2s Silence duration. It's burst potential is too high with Pitch Perfect attached? Whatever, remove the Pitch Prefect proc from Nocturne and put it on Blast Arrow instead. The nerf to it no longer doing damage is just petty. I'm sorry that they thought 2000 dmg was a deal-breaker.

But don't try to peddle B.S. when RDM's line AoE Silence Resolution is untouched at 25y on a job that actively has to get into close range to function properly and has no penalties on the range of their ranged skills. How will players counter such a difficult ability? Nobody knows, but I guess they'll have to find a way, but in the mean time BRD's single-target 25y Silence range needs to be nerfed. It's ridiculous, BRD's filler attack requires them to be at 15y or further from the target for max damage; they clearly designed BRD to try to keep its distance, but it has to move-in to exactly 15y or closer if it wants to use its only good burst window? But RDM gets to keep it's long range (and damage) on their Silence despite being better equipped to stay in close-quarters? lol If they have to be closer than RDM, they should have at least kept their 3s Silence.

To be honest, the whole things stinks of whoever is in charge of PvP balance getting bullied by a good BRD player one too many times in CC and deciding that BRD shouldn't be allowed to secure kills at a range anymore the way other ranged jobs still can as a salty kneejerk reaction.