Ditto. Or even an old-school LED that indicates data transmission like the HDD LED in old computer cases. Make it in hardware so that if any data is sent through the wire, it lights up.
I think you are confusing this with data tx/rx lights for exfil[0]? I've never heard of a tempest attack (that is, revealing the data written via a side channel) on a hard drive light.
Old? I'll admit I haven't changed my case in a couple years, but mine currently has one, and I can't remember the last time I've used a desktop computer that didn't have one.
