e.g. by default this will trigger IDE0300:

IEnumerable<int> source = new int[] { 0, 1, 2 };

but if that's changed to:

IEnumerable<int> source = [0, 1, 2];

the compiler tries to protect the readonly-ness of the enumerable from upcasting and emits something along the lines of:

IEnumerable<int> source = new ReadOnlyArray<int>(new int[] { 0, 1, 2 });

which is then both an extra allocation and hinders some downstream optimizations from code such as we have in LINQ that tests whether the source is a T[].

Putting this into the .editorconfig prevents the IDE from recommending switching to collection expressions in such cases.