they're stored as a spritesheet in the tf2_textures_dir.VPK as materials\hud\dneg_images_v2.vtf
To open the VPK, use GCFscape
To open the VTF, use VTFedit
If it appears as a blob of weird colours, that's because the shapes of the killicons are stored in the alpha (transparency) layer. Just go to view -> channels -> select A
There won't be anything hinting at a new update in it
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