Why do you say that? I would say the opposite.
Ruby isn’t making all strings immutable here. Just string literals. You are free to allocate mutable strings that can be appended to, to your heart’s content. It is extremely rare that modifying a literal is intended behavior, since their contents are permanently persisted throughout the lifetime of your program. With your example, this would be like having one shared global buffer for your final document.
Ruby is not a web focused scripting language.
JavaScript is much more of a "web-focused scripting language" than Ruby is, and it is quite happy with immutable strings (only).
> I think the comment is about that you now need to choose mutable vs immutable, and that is framed as a consequence of broader adoption.
Ruby has also had immutable (frozen) strings for a very long time, so you've always had the choice. What is changing is that string literals are (eventually) going to migrate from "mutable with a strong-encouraged file level switch to make them immutable" to "immutable".