Tin cans, once opened, would start having a chemical reaction with the food and air that would transfer the metal taste to the food. You can still experience that but most acidic foods now have a coating on the inside to prevent such.
I'm not aware of any misguided fear of Botulism, though I am aware of a cavalier disregard for the seriousness of it because, like lightning strikes, your chance of it is still quite rare....but it does strike.
You won't find Botulism in an open bottle of ketchup because its a high-acid food product and usually has bunches of food preservatives in it. Homemade products without the preservatives will see higher instances of bacterias, molds, yeasts and fungi, if not kept properly refrigerated. Conditions that favor botulism include a high-moisture, low-salt, low-acid environment in which food is stored without oxygen or refrigeration.