My ABC local radio drive presenter Richard Glover hasn’t used shampoo or any other product in his hair for a while now. He did a “no shampoo challenge” few years ago and encouraged his listeners to join him.

You might find the following interesting:
The No Shampoo challenge - Richard Glover - Opinion - smh.com.au
UPDATE! Is shampoo a ripoff? The Final results are in - ABC Sydney - Australian Broadcasting Corporation
or just Google “Richard Glover no shampoo challenge”. There’s lots of stuff out there about it.