waveform – Relaxation oscillator giving distorted square wave

The 100 ohm feedback resistor (Reference Designators!) is so small that the right-side opamp (Reference Designators!) output is going into current limiting. Increase the 100 ohm resistor by 100 x and decrease the timing capacitor (Reference Designators!) by 100 x.

The TL071 output is characterized for output currents up to 50 mA. If the load impedance is so small that the output needs to supply more current than that, the output stage protects itself by reducing the output voltage (and thus reducing the output current). This is not a tightly-controlled parameter; it is a statement about the limits of what the opamp is guaranteed to do. The actual performance is greater than this to cover variations in individual devices due to manufacturing differences.

In your case, during the rapid vertical transitions the timing capacitor appears as a dead short to the reference voltage. Based on the slope of the waveform, my guess is that the current limiting kicked in at around 83 mA. Since here probably is no reason for such low impedance timing components, adjusting them such that the output no longer has to supply such high currents should solve the problem.