For more, see here: http://www.leftbrainwave.com/2017/01/the-scriptural-sanction-for-caste-based.html#TOC5c2g

From my blog:

With regard to Valmiki, the story regarding his beginnings is controversial, to say the least. The popular story that he was a low-caste highway robber is not well-corroborated in scripture. Most versions of the Ramayana do not mention this.

For instance, the translation of the Ramayana by Manmatha Nath Dutt (Volume 7, Uttara Kanda, Calcutta, 1894, page 1909) mentions that when Valmiki brings Sita and the twins Lava and Kusha to Rama's court to prove Sita's innocence, he says, “I tell you the truth, O Rama, that these irrepressible twin brothers are your sons, O descendant of Raghu. I am the tenth son of Pracheta, and so, far from speaking untruth, it does not even spring up in my mind. I have performed austere penances for many thousand years; I now swear before thee, that if this Maithili is found touched by any sin, I shall not reap the fruit of my ascetic observances extending over many thousand years. I have never before perpetrated a crime, either in my mind, body, or speech. If Maithili is found divested of sin, I may then partake of the fruits of piety.”

This passage does not support any suggestion of Valmiki having been a highway robber in his past — his clear assertion that he had never before perpetrated a crime, either in his mind, body, or speech, puts paid to that speculation. Also, the way he states his birth (as the son of Prachetas, and therefore speaking untruth never even springs up in his mind) clearly indicates that he is high-born, and the son of a Brahmin.

The question of whether Valmiki was a highway robber before becoming a sage has been hotly debated, and has even been the subject of a court case. I quote some relevant portions of the judgement in this case below:

A debate rages on amongst scholars both religious and academic about the authenticity of the story, whether Maharshi Valmiki was a dacoit. The actual facts appear to be lost in the mists of antiquity. Dr. Manjula Sehdev, one such scholar kindly consented to address this court. She has carried out a detailed research into the origin and authenticity of this story and if her work is to be accepted as correct, there may be force in the argument raised by counsel for the respondent that there is no historical or mythological basis for the story that Maharishi Valmiki was a dacoit before he became a sage. It would, therefore, be relevant to reproduce a few salient features of her research.

1. From Vedic literature upto 9th Century A.D., there is no reference as such that Maharishi Valmiki led a life of a dacoit or a Highwayman.
2. Even upto 9th Century A.D., the etymology of the word Valmiki (a person born from an ant-hill) is not available.
3. In his own work 'Ramayana', he is called Bhagwan, Muni, Rishi and Maharishi. No reference of his Highwaymanship is available there.
4. First reference regarding Rishi Valmiki as a Highwayman is mentioned in the Skand Purana. The time of this Purana is considered 10th Century A.D.
5. The first reference of the mantra 'Mara-Mara' has been mentioned in the Adhyatma Ramayana (Ayodhya kanda, 6.80-81). The time of Adhyatma Ramayana has been considered 15th Century A.D. by scholars. In the same way, we find mantra 'Mara-Mara' in Ananda Ramayana (Rajya-Kanda, 14.141). This work has been considered of 16th Century A.D.
6. The Bhakti movement was started in the southern part of India about 8th & 9th Century A.D. by Alwars. When this movement came at its peak from 3th to 16th Centuries A.D., many stories were woven around the personality of Rishi Valmiki keeping in view the importance of Sri Rama as an incarnation of Vishnu.

The salient features of this research appear to cast a doubt about the authenticity of the story that Maharishi Valmiki was a dacoit.

But even if Valmiki were a low-caste person, it is not relevant to the message of the Ramayana. Even if one were to assume that he was a Shudra by birth and reformed through Narada’s intervention, the story of Shambuka mentioned earlier clearly indicates that his low-caste birth has nothing to do with the message of the Ramayana, because in the story of Shambuka, a man is executed simply because he chanted mantras and performed penance in spite of being a Shudra — something that Valmiki himself clearly did quite regularly.