There's some controversy about the use of castille soap as a shampoo. For example, popular YouTuber Kim , of KimmayTube tested the ph of Bronner's Castille Soap and found it to be much higher than the recommended range for hair (between 4 and 5). You can watch her explain her findings.
She also did an informative series fraught with good visuals on understanding the effects of ph on hair.
That said, I've been using diluted castille soap successfully for YEARS, but I've never tried Bronner's Castille Soap. I had a bottle of Trader Joe's Castille Soap.
Turns out Trader Joe's formula includes Aloe Vera --which has an acidic ph---probably bringing down the overall ph of the product--perhaps balancing it for hair. The manufacturer recommends it's use as a shampoo right on the bottle's label.
I did not get a chance to personally test the ph of my bottle, because by the time I noticed Aloe as an ingredient of my castille soap, the bottle was empty and I was on my way to Trader Joe's to buy a new one. I was very disappointed to learn they are no longer selling their brand and promoting Bronner's Castille Soap instead. My bottle lasted for years--a little bit diluted yielded a rich sudsy solution. I'm sorry to see the product go. The representative had no information about why they had stopped selling it.
I can't recommend Bronner's, because I've never tried it--but I've read that some people report success using it diluted as well. You may want to test it for yourself and see how you like it. Test the ph for yourself, and if it's not in a safe range for hair, I'd leave it alone.