A nerf is on the League of Legends PBE for Heimerdinger that takes a hard swing at his upgraded turret's beam damage.
The nerf that's being tested for Heimer was explained in greater detail through a Gameplay Thoughts post, the final one of the year that touched on several different champions like Tryndamere, Master Yi, and Evelynn. For those that follow the PBE changes before they go live, the nerf to Heimer's ultimate turret beam damage certainly looks "hefty" as Riot Meddler described, and it likely will be if it goes live without any other changes.
"We've got a nerf in for Heimer that looks pretty hefty on paper, with his RQ turret's damage dropping from 200-340 (+0.7AP) to 100-180(+0.7AP)," Riot Meddler said. "It's likely to be pretty hefty in game too, which we feels appropriate given how well Heimer's performing at present, particularly his ability to make it really difficult for many champs to all in him during laning phase."
Rune changes or not, Heimer still persists as one of the game's most frustrating champions to engage on whether you have extra backup or not, so the nerf is likely one that won't be challenged my many of Heimer's opponents. However, it's important to point out that this nerf is only targeting the beam damage of the upgraded turret, not the power of the turret's individual projectiles.
"Worth noting though that this isn't a halving of his R's power," Meddler continued. "That's a nerf to the beam damage only, so the R turret's regular shots will still do the same amount of damage as at present. The ratio's unchanged too, so damage later in the game will be less affected. While the RQ is generally Heimer's go to choice of upgrade, other R uses are unchanged so situations where those are good won't be impacted either. It's still possible despite those things that this will hit Heimer too hard of course and we'll give him a look after 8.1 goes out, see if he needs adjustment in 8.2."
The nerfs are expected to go out in the first official League patch of the next season, but look for more changes in the next patch if they're necessary.