A friend has some old ones that he wants to use for erosion control, but they have to be cut to lay flat first. They are several inches thick and of course heavily reinforced with wire or cable.

I'd think popping some cinder blocks under the track. Then using some carbide demo blades with a heavy duty sawzall. Maybe some water near by incase it starts to warm up.

Angle grinder on the exposed metal once you get there.

See if the local fire department wants to practice with their jaws of life?!

Angle grinder with slitting disc(s)

Smokey and stinky, but effective and by far the easiest/fastest.

EDIT Just occurred to me that a circular saw with a metal cutting blade should work and would be faster and (possibly) less smoke. Go slow and let it take little bites of the steel cords.

Those tracks are a bitch to move around so I understand your friends dilemma.

If it were me and the tracks are off the skid steer I would take a cutting torch to them and make sure he is upwind when we does it.

These things don't have master links? Maybe call or stop by your local skid steer dealer and ask their mechanic how they change them out, maybe check Youtube?

Partner saw if you have access to one… for the person that asked, there is no master link in them, you collapse the idler pop track on then pump idler back up with grease

Ok I've never done it so there is that.
But of the things I own I would first pick a gas powered cutoff saw with a carbide blade.
Then probably a sawzall with the correct blade.
Then probably an an angle grinder if I had one big enough.
Then probably a circular saw with a metal cutting blade.
But realistically anything that can deal with the thickness (I have a CAT and its under 3 inches and I doubt any other brand is thicker) would be fine.

On that thick rubber, I would definitely not use any blade that cuts by abrasion/friction. Burning rubber will not only smoke and choke you out even if done outdoors (under no circumstances should you do it indoors), but might prompt 911 calls from your neighbors (who would reasonably assume something is on fire).

We cut heavy rubber conveyor belt with a diamond blade circular saw, cooled and lubed with dish soap, after much trial and error.