The torque on an induction machine is simply proportional to the difference between the speed of rotation of the magnetic field and the speed of rotation of the rotor. That applies at any speed - positive, negative, zero or anywhere in between. The rotor doesn't care or know any different. As m_c says, there's a thermal issue if you apply high torque at low speed for any length of time, so machines intended for that sort of use tend to have separate cooling fans or water jackets.

The faster the motor spins, the greater the back emf is. But torque is proportional to the current in the stator. To drive current into the stator, you need a voltage difference but once the back emf reaches the DC bus voltage within the VFD, you run out of that driving voltage. This is the base speed and without clever control (field weakening), you can't go any faster. It's similar to when you connect an unloaded brushed motor across a battery - it reaches a speed where the back emf equals the battery voltage.

The back emf voltage constant is Kv and is proportional to the magnetic field strength, so field weakening extends the speed range by suppressing the magnetic field generated by the rotor in an induction machine and hence reducing Kv. In a PM machine, it does this by creating a field to partly cancel out the field from the magnets in the rotor. You don't get that in normal VFDs but it's used in EV drives to give an extended speed range.

If you were talking about permanent magnet machines such as servo motors, the same principles apply in terms of torque being proportional to current and back emf being proportional to speed but now the torque generated is proportional to the positional angle between the rotating field and the rotor. Without a closed loop position control, a permanent magnet machine driven with a static field feels like a torsion spring as you try to move away from the no load position. You see a similar angle error / torque relationship with open loop steppers. If you have a stationary field in an induction machine, there is no torque at zero speed but it increases as you turn the shaft faster. It's like stirring a bucket of treacle - rather weird.