Can a pole be reached for a low or high pass filter? How can $\hat{s}=-\frac{1}{RC}$?

Do you mean, ``Can the complex frequency actually take on the pole value?''. In our examples today, we were dealing with only pure sinusoids, which have purely imaginary $\hat{s}$, i.e., $\hat{s}$ values only along the imaginary $\omega$ axis in the complex frequency plane. Under these assumptions, we never have a signal with a frequency at the pole of a low-pass or high-pass filter.

(Complex frequencies with non-zero real parts describe transients, and are useful for understanding of ``impulse response'' of a circuit. However we will not be covering that in this course.)