Is the roll-off gain of filters always -20 dB/dec?
For an ideal first order filter, 20dB/decade is the asymptote in the stop band.

A real filter might do other things far into the stopband, due to non-idealities in the components. For instance with an RC filter, stray C across the R, or ESR in the capacitor, will both limit the ultimate attenuation.

A second order filter will go off at 40dB/decade. An Nth order filter will drop at 20N dB/decade.