I think you're referring to deionized water. Deionized (DI) water is water that ...

dwighttk · on Sept 28, 2019

Outside of hydroxide and hydronium, which ions are left with “pure H2O (no minerals etc)”?

And DI water gets hydronium and hydroxide pretty quickly (if they ever go away.) H2O just breaks down into OH- and H3O+ on its own (as far as I know)

hollerith · on Sept 28, 2019

I agree: in fact the deionization process adds one hydronium or hydroxide for every (mineral) ion it removes:

>Deionization is a chemical process that uses specially manufactured ion-exchange resins, which exchange hydrogen and hydroxide ions for dissolved minerals

https://en.wikipedia.org/wiki/Purified_water#Deionization

In other words, if deionized water is unhealthful because of a lack of ions, distilled water is, too, because it doesn't contain any more ions than deionized water does.