Well, I think how prototype works is much more important than using React.
I think having an interview in JS without knowing prototype is like having an Java interview without knowing polymorphism, abstract class, interface or something.
Although I'd also note that in practice people tend to use a subset of JS in their work, and often prototypes aren't directly used. (I understand they power everything under the hood but I'm talking about something product engineer has to think about.)
If you are just building frontend applications using frameworks like React and Vue, you don't really need to know how things work under the hood. Especially with all of the new syntactic sugar introduced to JS in the last few years.
But, if you are building lower level tooling and you want to understand why things are implemented the way they are (this post being a great example) you really should understand them as best you can. A lot of design decisions in JS are influenced by these lower-level design "constraints."
However, when it comes to a mid or senior level engineer, it's important not to just make things work, but to understand the how and why in order to build performant applications at scale.
So to answer your question, no it's not a necessity to build applications. Yes, it is if you want to excel at building applications.
Prototypes are more powerful than anything Java has to offer.
It's a low-level concept that is mostly used to create a class system on top of it, rather than used directly.
Polymorphism, abstract classes or interfaces are part of the day-to-day stuff a Java dev has to meddle with.
You can absolutely be a great javascript developer without ever touching prototype. Prototype is an infection of object-oriented programming into a functional language.