Is the claim that they produce less particles, or that they distribute less particles because the shoes/drum is more of a closed system? Generally drums are worse brakes than disks so if they don't brake as efficiently, you'd be grinding them for longer, likely making more particles.

I haven't heard much about drums lately except that cheap cars still sometimes have them in the rear.

Right. The basic idea is that because new cars will be hybrid or electric, most braking will be regenerative, so drums are fine.

That makes more sense, if the fronts are regenerative, that's where most of the braking force comes from so I can see drums being sufficient in the back. A lot of lightweight motorcycles (especially classic styled ones) have rear drums.

The only thing I hate about driving a front disk/rear drum car is how deep the brake pedal is before you feel it activating the brakes. Going from responsive brakes to "oh shit they aren't activating yet..." panic for a second can take some getting used to!