Erm... just about everywhere in the tabletop fluff since forever, plasma weapons have been touted as marine killers. Melta less so since they hype up its use against armor - meaning vehicles, probably because its easier to sneak up on a tank than it is an astartes... I guess the high AP and DMG kinda cover this, but felling on one or both weapon types certainly wouldn't feel wrong to me.
Plasmas and meltas aren't "marine killers". They're EVERYONE killers! They kill marines, like they do everything else, by heaping on damage dice and burning through armor. Marines should be MORE likely to survive a plasma shot than a mortal, and thus felling would not serve the correct purpose. From a balance perspective, adding Felling to plasmas and meltas would boost their power significantly, putting them leagues ahead of everything else. Which, since they're already leading the pack, is not a good thing. Not for players exposed to them, and certainly not to any NPC the GM wants to threaten them with.