The appearance of humans rapidly declines over the course of the series, implying that most or all of them (besides the mysterious Agent who appears several times) have been killed off.

The Skibidi Toilets are also very aggressive, with many episodes ending with them attacking and eating the camera, which is presented as a first-person view, implying that they eat people.

Skibidi Toilet 4 shows people sitting on toilets before a Skibidi Toilet bursts into the room, at which point the view zooms out to show that all the people are now Skibidi Toilets, suggesting this is possibly how they reproduce. (Edit: I misremembered, the view doesn't zoom out. It focuses on the Skibidi Toilet as it approaches the camera but it pans past the people who have turned into Skibidi Toilets and are now ominously staring at the camera.)

Thus, it's very likely that there's no human plumbers left to do the job.

The main protagonists of the series, the ubiquitous Cameramen, are sometimes shown using plungers as weapons, however.