I'm sorry if this has been posted before, but I couldn't find it here.

I'm new here, I've just discovered KDenLive, and it seems like the perfect FOSS Sony Vegas alternative, or at least as close as you can get on Linux...

Anyway, I'm wondering, does KDenLive have a CAS (Content Aware Scaling) effect? Is there anyway I can achieve this? I can't seem to find such an effect on the program, is there any chance it might be added or is there already a way to do it using KDenLive (or another Linux program)?

No kdenlive does not have this. Google tells me this is a photoshop effect. Which implies to me that it is an effect for still images. Do any video editors out there supply this effect for moving images?

Yeah there are all sorts of videos using this feature, just searching it up gives you a bunch of results.

Well the first one I found on your search in youtube pointed me to this tutorial on how to do it.
And it says

Tools needed:
•Your video editing software of choice (and some knowledge on how to use it!)
•Neil Cicierega's content aware scale script http://neilblr.com/post/42948042669
•photoshop cs5/6, make sure you have quicktime installed too or you won't be able to import video into Photoshop

So it seems to me that yes - sure kdenlive can do content aware scaling just as well as other video editors - because you use photohop to make the CAS images and import them into "Your video editing software of choice".

So think my hunch that this is a photoshop-still-image-thing is correct.