I'm not aware of any changes we made there. Assuming we're talking about the same thing of course, each line of the killfeed has a faint gradient bar behind it and has done all the way back to RO2. We just made it much more transparent. Most of the time I can't even see it and really it's only to make the feed slightly more visible against certain background colouration. It's not customisable from memory - though you can adjust the opacity of the whole killfeed and it probably is affected by that slider (albeit at the same time as the text opacity).