To me a reactive data source would be something like Asana, Trello, Teamwork or on an ecommerce site it could be the cart widget. Basically any type of data that needs to react without requiring a page reload or any data source that needs to sync with another users inputs in real time.
RxJS
aka the reason so many websites take 100MB of memory nowadays
Don't use it
It fairness you can do reactive data without RxJS. For example by using the vanilla web socket API. Agree that RxJS is best avoided though.
I got into RxJS cause I had to deal with an Angular project a few months ago, what's wrong with it?
You can react to state changes with an astonishingly small amount of code with JS [0]