Diathermy or Radiothermy has been around for a long time. Using substantially lower power than a microwave oven, a radiothermy unit warms subcutaneous tissue by causing water molecules to generate heat by friction. (Specific frequencies called waterlines excite water molecules to align with the wave. Since the wave alternates 1-5 Billion times per second, the water molecules rub against each other and generate a lot of heat proportional to the energy being transmitted. This allows the warming if internal tissue).
Unlike a microwave oven, diathermy / radiothermy energy is usually inducted via a small antenna that sits over the tissue to be warmed, but at substantially lower power.
On the other hand, if you want to remotely drive someone nuts, or create buzzing or voices in their ears, you could repurpose a microwave oven’s magnetron tube by adding a high gain directional antenna, and an input to modulate the microwave output and point it at them, creating your very own Havana syndrome generator.
Or a very long range audio transmission system.
(the moniker “Havana Syndrome “ may be new, but the technology/ capability is old)