Edit - changed previous sentence to "proptype hint". I originally erroneously read the parent as saying "prototype hint" which I couldn't find results for, but even after correcting my reading error my question still stands.
PropTypes are used for a combination of debugging during development ("you accidentally passed a string instead of a number", or "you forgot to include a required prop"), as well as documentation for a component. React devs have traditionally used PropTypes to act as readable documentation of a component's expected props, and there are tools that can extract PropTypes usages and generate documentation files.
However, with the rise of TS and Flow as static type systems, the need for a runtime-based type checker has gone down, especially since people using TS/Flow have probably already declared types for a given component's props.
Ie. ‘lite’ type checking, used in react.
(did you perhaps see the results for ‘javascript prop type hint’ as a google autocorrect suggestion or something? This is a well known term, but I’m pretty amazed two seconds of google didn’t discover what it was...)