You can buy shares of all companies where the NRI/PIO holding of the paid up capital is below the RBI prescribed ceiling. The RBI prescribed ceiling ranges between 10% to 24% of paid up capital. Further an individual NRI or PIO cannot hold more than 5% of paid up capital of any given company on Repatriation basis.
The list of shares not available for NRIs and PIOs investment is updated in the RBI website here.
Based on the updates in the RBI website we accordingly enable / disable investment in the shares on the Trading System. However, NRIs trading in NRO account can buy stocks like a Resident and this restriction is not applicable to them.