Many new cars have start/stop features builtin. If the computer controller detects that the engine may have trouble starting (low temperature, low battery, starter motor failure, whatever), it won’t stop the engine for you.

Or that’s the theory.

It’s not necessarily an issue of fuel, but the overall wear on the components and engine when you start a car. A starter motor only has so many “starts” it can do before dying. The battery too.

Starter motors have gotten a lot better since the “bad old days” and engines start more smoothly thanks to fuel injection and computer control systems, so manufacturers have decided that it’s ok to start/stop engines as needed, but the reason for not doing it was never a matter of fuel savings.