[2023] What is this box that shows up right after he dies to the omen?

Not WallHacks.
Omen is standing in front of a wall decal in the map that renders above VFX (For some reason, thanks Unreal).

If you go on Split and look where Omen is standing, you'll see some sort of wall panel that has this exact look.
Creds: VFX Artist here, I've been fighting the maps team's decals for years lmao, it isn't a new issue.