What frequency and voltage should the electrical outlet have in order to be the least harmful to humans and still be useful to modern appliances?
This is rather like the question, "and what speed and altitude should airplanes fly, in order to be least harmful to humans while still being useful for travel?" I think any flight profile that couldn't cause injury in case of a crash would be no better than walking. If we limit altitudes to 1000 ft, are you any less dead when the airplane hits the ground? It's better to focus on making the airplanes not crash.

Your graph only goes up to 100mA. That's not very much current. Say you want to power your 60W laptop; the voltage would have to be:

$$ V = \frac{60W}{100mA} = 600V $$

Say you want to run your 1kW popcorn popper:

$$ V = \frac{1000W}{100mA} = 10kV $$

So, if you want to design an electrical power distribution system for the home where the current is limited to a value where one could let go under any condition while still having enough power to run common kitchen appliances, the distribution voltage would have to be at least 10kV. That would bring on a different class of problems.

The fundamental problem here is that electricity in the home can deliver a lot of energy. A typical 15A circuit at 120V is 1.8kW, or 1800 joules per second. For comparison, the kinetic energy of a bullet fired from a .45 handgun at the muzzle is about 500J. In terms of energy, each circuit in your house has the potential to fire about 3.6 bullets at you each second. It's hard to make that safe under all conditions, any way you slice it. 1.8kW, be it from thermal energy, kinetic energy, RF energy, can probably kill you, one way or another. All we can choose is how.

So unless you want to limit the power of electrical distribution (which would severely limit its utility), the problem is just putting safeguards around whatever problems there are so that 1.8kW never flows through a human. For example, a GFCI is designed to interrupt the circuit at very small fault currents. And there are the obvious methods of protection: insulation on the wires, outlets designed to prevent accidental contact, etc. If you want to improve safety, I'd focus on these things.

A notable case study is in the electrical distribution system used in airplanes. This runs not at 50 or 60 Hz, but 400 Hz. It can often be heard as a whine when the crew is making announcements. But here the goal isn't safety, but weight savings in the smaller transformers required.