wallpaper in the Kitchen

I would say it will depend on the cooker, most modern cookers discharge the hot products from the oven to the front, while old cookers used to discharge them out the back even scorching the wall in some cases, not many of those around these days but certainly worth checking.
Goes without saying that wallpaper behind a hob without protection would be a no no!

In practice tiles that are resistant to scorching and easy to clean are the best solution.

 

Better to have tiles, as it's easier to keep the area clean.