I have a bench variable-voltage power supply. Mine is a very low-end one, doesn’t even have an integrated voltmeter, so you need a multimeter or oscilloscope or something to set the voltage.

But…if you need some kind of exotic power at a given voltage and don’t have time to wait for an order to come in, one of those will fill the hole for all kinds of fixed-voltage power supplies.

I have only very rarely needed it. I’ve used it to provide 5V power to a PIC project that I was breadboarding, and another time tried checking whether it was low voltage causing a problem with a given battery-powered device. Maybe one or two other occasions.

If you specifically want barrel plugs, you can get “universal” adapters that will adapt one cable to various diameters and polarities.

I think that USB is also kind of increasingly filling the hole here as a universal standard for DC power on newer products.

19.5v is pretty common for stuff that needs more power, but I’d bet there’s some threshold at 20v that means a device is no longer “low voltage”.

But AC? Are you sure? That would be exceedingly unusual.