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Earth will be devastated.

I'll try to give you a highlevel rundown of what would happen.

Rapidly vs. Slowly

Yes, the effects would be substantially different. Depending on the speed of fading the effects on earth would become increasingly stronger. The worst case being the dissolving from one second to the next.

Important factor: The Sun

Sun's activity plays a major role for the effects on earth after the magnetic shielding is gone.

Solar winds are the main reason why we need the magnetosphere around us. It protects earth's atmosphere and therefore all life on earth. Otherwise we would be exposed to high amounts of radiation (like in: radio active radiation; but probably worse, because nearly constant and in high, lethal doses).

However, the amount of particles the sun emits is not constant. It fluctuates. Sometimes there are solar bursts or storms, throwing large amounts of highly charged particles outwards flying at high speeds. Sometimes solar wind is "relatively" calm, but still deadly.

Effects on Earth (from high to low)

Let's assume the effect of dissolving is over, meaning either it took a while or some seconds and now the magnetic shield is gone.

Satellites would suffer first. They are most vulnerable because of their high-tech components. Solar winds can induce high amounts of current into the electric circuits, causing overloading, short circuits or melting of delicate components, leaving the satellite eventually disabled and adrift in space.

Solar winds will also start to push the satellites slowly but steadily out of their orbits (over the course of centuries probably, depending on strength).

Next being hit are the astronauts in the space stations ISS and CSS/Tiangong. They have protective installments but they are not sufficient against the unbraked storm of particles.

Afterwards the atmosphere is hit by the solar wind. This will cause the atmosphere to erode over time. It will be "carried away" or "ripped off" the earth piece by piece and dissolved into space.

This process is again heavily dependent on the sun. If there are no superficially increased levels, it will probably take decades or centuries until the effect on the surface will start to become inhospitable.

On the surface, gradually with the decay of the magnetosphere, there would be heavily disturbing lights all over the sky, also at night. Very similar, but stronger and brighter, to the Aurora Borealis/Australis (north/south pole). This will start to disturb life on earth already significantly. Nights would never be dark again.

If the ozone layer is gone, UV radiation from the sun will also draw its tribute from earth. Atmosphere in general will thin out and get unbreathable, higher amounts of winds/storms are probably induced, airplanes will not be safe to fly in, temperatures will rise globally, plants and crops will dry out, water will vaporize at some point, humans and animals will suffer from the high radiation levels, famine rises, the death toll will be extreme. Some counter-measures will probably be possible (bunkers), but not sustainable forever.

Since you asked specifically for humans: They will suffer like those poor souls exposed to the uncontrolled radio active fires of Chernobyl back in 1986. At a certain point this will just be gruelsome. Radioactive burns and poisioning and almost instantly lethal doses.

Eventually earth will be a barren rock in space. Similar to Mars. No magnetosphere, no atmosphere, everything radio actively contaminated, no life.

Van-Allen-Belt

The Van-Allen-Belt is the inner most part of the magnetosphere around earth. It is considered especially malicious to high-tech instruments in lower earth orbits.

What you have maybe found was the notion that the Van-Allen-Radiation-Belt is considered a nuisance and there are considerations and even plans to remove or deplete it. Wikipedia has it that it is suspected not having any effects if the innermost part would be drained (-- I doubt that heavily).

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