Also note that "if it's left in gear" is virtually irrelevant in North America, fwiw - something like under 2 percent of cars here are manual. Vast majority of vehicles and models do not have it as an option.
Before my life in tech I worked in car audio installs back in the 90s.
One day one of the senior installer techs put a remote start in his own Mazda truck which had a manual transmission. A bit of time went by and we were outside smoking one day when he must have hit the button in this pocket. Apparently the e-brake wasn't on, and the truck was in first gear, so it started and drove across the parking lot and hit the wall of a building as we all watched.
That's the key part. Proper :)
IIRC he just bypassed the sensor that would have been for Park on an automatic, since there were no manual transmission kits for sale as they were (and I think still are) illegal for sale where I live.
He would even refuse to do this sort of thing for other people so it was pretty funny it happened to him.
And as we also have pretty tiny engines here, it takes a long time to heat up this way because first the cooling water needs to get warm which takes ages with the engine running stationary. So I only did this once in a while if it was really needed.
My old Volvo did have an option to open all the windows with the remote though which was great to let the heat out in summer.
“Vehicles fitted with catalytic converters emit most of their total pollution during the first five minutes of engine operation; for example, before the catalytic converter has warmed up sufficiently to be fully effective.”
I don't there is a big benefit overall.
In my climate, on a 45c day, inside the car's going to be more than that. It's definitely a workout for the generally imported car's A/C unit. I am guilty of turning the car on and waiting under a tree or something for the AC to bring the temperature down to at least ambient.
I'm looking at a series hybrid car. I must admit, pre-conditioning the car off the battery would be pretty luxurious.
Yeah, it's getting more and more difficult to even buy them at all over here. Rather slim pickings.