To me it sound like it is possible to shift only one gear at a time and only when pedaling. So if you stop at traffic light with high gear, you can not switch to lower gear while waiting for the light to switch.

Most chain drives can't switch gears without pedaling either.

Internal gear hubs typically (always?) can change gears when not pedaling, derailleur gears can't.

And most chain drives are derailleur systems, not IGH, so you're both right :-).

And most IGH systems are chain drive as well, so all three of you are correct :-)

if chain includes belt.

My experience with Shimano gears is that if I change when stopped or while not pedaling the change completes when I start pedaling again. The only important thing is to go easy on pedals if starting from still and don't change both the front and the rear gears.

As a category of bikes that also can’t do this is quite popular this isn’t a deal breaking issue.

You definitely can, just hold the front brake and push the handlebars forward to lift off the rear wheel. It's even easier if you're using clipless pedals.