Unfortunately, it's not that telesto was nerfed, it's that they lowered damage across the board for all energy slot weapons, and buffed kinetic slot weapons. Don't ask why.
As of last Tuesday?
You aren't talking about the 2.0 Forsaken rebalancing, are you? Special weapons that used to be heavy weapons got a damage reduction a week before Forsaken with 2.0. Except Telesto. Kinda...
Telesto had its bolt damage reduced just like all fusion rifles, but the explosive damage (where most of its damage came from anyway) remained untouched. That's why it was so damn good. It was a heavy weapon in the energy weapon slot using greed ammo. It killed bosses faster that a rocket launcher.