From the patch notes with the change:
Second, we've also updated the way this thing builds stacks in the first place. A simple change is that Devourer builds on monster takedowns (assists or kills), rather than just kills, so you don't feel bad when you give away that blue buff, or when Kalista secures the Dragon with a mega-Rend. Also, speaking of Dragon, we're really trying to incentivize Devourer junglers to focus on contested jungle objectives, like the Rift Scuttler or Dragon, as it gets them out into the sunlight (can't be playing League all day) while also giving enemies clear ways to slow your Devouring roll.
LARGE MONSTERS 1 stack per kill ⇒ 1 stack per takedown (kill or assist)
EPIC MONSTERS 1 stack per kill ⇒ 5 stacks per takedown
CHAMPIONS 2 stacks per takedown ⇒ 1 stack per takedown
RIFT SCUTTLER 1 stack per kill ⇒ 2 stacks per takedown
As you can see, their example calls out by name giving away blue, but the code itself is all generic and about the takedown (kill or assist).