See #31913 and #30499 (comment) for some more info

(TLDR; it's about "array response bodies", not JSON per se)

JSON stands for JavaScript Object Notation. No API should respond with anything else than an object and/or an array. Yes, this is a should. toArray works with conforming APIs, and will throw a well-defined exception when something else is returned. When this happens, one should use getContent() instead.
The links @ro0NL shared are relevant to get a better understanding of why/how things are designed this way.

Also, we're adding more ->to...() methods, such as ->toStream() added in #32290.

Maybe getJson is bad name also, but better then toArray for method that can return NOT ARRAY.
e.g var_dump(json_decode('true', true));

I'm about naming. not else. (but parameter for return object will be fine too 😄 )

but better then toArray for method that can return NOT ARRAY.

it will always return an array, or error as such. That's exactly the main-feature of this method.

Nope. It just do json_decode on ANY DATA

Data can be simple "json encoded" string for example. Like "example"

see a few lines below :)

Oooops, missed it )