Is the frequency response of a circuit only valid for steady state (phasors)?
Yes, it assumes the transients have decayed to zero.

But note that the frequency response contains information on the transients, this is why it's so useful. It's a steady state measurement that gives transient information, so filtering etc can take place in a steady state environment which, potentially, gives more accurate results.

Furthermore, SS frequency response can detect transients that may not even be visible on, say, a step response.