Honestly it’s weird how upset people are about this.
Sure there are some foods mentioned, like fresh pizzas, puff pastry or cake baking where you can get worse results if you don’t preheat the oven.
But anything from the supermarket that is just being reheated/finished off (steady) in the oven, putting it in from cold or frozen it really doesn’t matter at all. Bang it in when you turn the oven on and just allow that you might need an extra five mins or so. Ovens can be all over the place in terms of temperature, even fan-assisted so you’re always going to have to allow for the packet being incorrect.
Obviously if you’re reheating something in a casserole dish from the fridge you have to put it in when you turn the oven on, otherwise there’s a chance you’ll crack the dish with the extreme temperature change.