Based on this line in bullet3/pybullet.c, I assume that the default contact ERP in Bullet is 0.005, at least when using PyBullet.
In comparison, ODE uses a default ERP of 0.2 (ODE user manual), although I am not sure if that is the general default ERP or only the default ERP for joints.
Why is Bullet's ERP 40 times smaller than ODE's?