Hmm... previous reply didn't show up...
Sooo... basically, check out TidyPlates\widgets\DebuffWidget.lua,
starting at line 670+...
The icon artwork is set when there's an active debuff, but the
CreateAuraIconFrame function sets up the artwork frame.
The important line is...
frame.Icon:SetTexCoord(.07, 1-.07, .23, 1-.23) --
obj:SetTexCoord(left,right,top,bottom)
... which crops off the default rounded border, and makes the image
rectangular.
I don't know of any unit frames that also does this. Actually, I've
gotten a lot of negative comments for the squished shape... lol.. but
I'm glad that you like it, too!