ATtiny85 pin as output, what is input impedance?
My question is: is it safe to allow a capacitor to discharge back into
  the ATtiny85 when the output signal is held low?




Whatever current is delivered to the capacitor is limited by R and whatever current is taken back from the capacitor is, again, limited by R.

Digital IO pins when used as outputs are cable of delivering (sourcing) current and taking (sinking) current. Find the data sheet and locate the section called electrical characteristics and there you will find the information you need.