Unclear recommendation "If you use locale data arrays, this API..."

[Flachdachs]

When using the Angular Update Guide for updating from 10 to 11 there is the following recommendation:

"If you use locale data arrays, this API will now return readonly arrays. If you were mutating them (e.g. calling sort(), push(), splice(), etc) then your code will not longer compile. If you need to mutate the array, you should now take a copy (e.g. by calling slice()) and mutate the copy."

What does "this API" means? What does it refer to?