My microwave has a sticker so you don't heat liquids without a teaspoon. I've heated milk in it every day for years, every time with a teaspoon in it. It also heats up faster with it.

I believe any smooth piece of metal without any gaps or sharp edges (so no forks or foil) is fine in a microwave.

It’s actually the recommended way to heat up liquids in a microwave, especially higher viscosity liquids that don’t mix quickly like a sauce. Since the spoon can transfer heat to the center faster than the liquid can by itself. The waves can’t penetrate far into the liquid and can’t heat up the core of a volume of liquid unless it’s spread out. Just make sure the spoon doesn’t touch the sides when it rotates.