In 5th Sarga of Bala Kanda of Ramayana, I have stumbled upon the following sloka (4th).
तदिदं वर्तयिष्यावः सर्वं निखिलमादितः | धर्मकामार्थसहितं श्रोतव्यमनसूयया || १-५-४
Being such a legend, we two wish to relate this Ramayana entirely and completely from the beginning, which is endowed with the values and means of probity, prosperity, and pleasure seeking... and this be listened without any caviling.
Sage Valmiki was mentioning about the Ramayana is about only 3 Purusharthas, ie., probity, prosperity, and pleasure seeking only - धर्मकामार्थसहितं, but not about Moksha.
Was Moksha (liberation) not considered as one of the Purusharthas in Ramayana period?