masklinn parent
By your incredible criteria of things being mutable strings if they behave nothing like strings but can produce one, JavaScript absolutely does have a mutable string, it’s called Array. It’s also a mutable integer.
I don't consider JavaScript's Array to count because using it as a mutable string incurs a significant amount of additional overhead, because it has to box every character and check at runtime the type of each element. This is not true of the APIs that I listed.
Ah yes, arbitrary and capricious goalposts moved randomly as you get unhappy with where your decisions lead. Truly a hallmark of solid thinking.