This is an issue that has been raised multiple times (#1735, #1745), and was one of the driving forces for converting the exercise from a class-based implementation to a funciton-based one.

If the mentor notes indicate that immutability is a requirement, they are wrong and need amended.

If the mentor notes simply state that immutability is a strong recommendation, that is perfectly valid; this exercise is an early core exercise, and is meant to be a simple practice in working with arrays. Immutability is a subject for a later exercise.

It is difficult for me to tell from the mentor notes what is a suggestion and what is a requirement for accepting the exercise. All the reasonable solutions use sorted. My assumption was that more was required than just passing the tests or the acceptance would have been automatic and based on that. I'll close this, read the other threads (I did try a search or two first, sorry about the duplicate) and look into clarifying the mentor notes.

If the mentor notes indicate that immutability is a requirement, they are wrong and need amended.

I wouldn't say the current mentor notes are wrong, just a bit too open for interpretation. I created a pull request on the notes.