To give you more context, when rescript was originally created, it used the OCaml standard library as its standard lib, but it was not really convenient on a JS backend, so a Js library was created which aimed at providing 0-cost bindings to JS APIs. Over time, idioms in rescript changed and a new standard library was created, Belt, which focused on rescript types more than on JS APIs. The situation was not ideal since people want to use standard libs without having to think if the underlying type comes from an OCaml or a JS background, so it was decided to create a standard library that would cover everything, Core was born and was later renamed Std once it got shipped with rescript itself as it main standard lib.
Now that the situation is clear in v12 - Std is the standard lib for everything in rescript, is here to stay and is shipped with the compiler - Js lib is getting deprecated. So try to avoid its use.