PCs have almost never supported CEC. PCs use a different signalling method to indicate to the monitor that they’re on/off.

That is sort of what sponsored segments are, and what sponsorblock is quite effective at dealing with.

If they’re injected on the fly at variable time points, it gets harder, but I suspect fingerprinting could work well enough.

TVs often do a bad job at switching on when the computer turns on, then off when it turns off/goes to sleep. Drives me spare. That was fixed in like 1995-2000 for normal monitors.