IMHO - 'Unused' Brass may get brittle just from age.

If you have unfired & unprimed Brass that is over 20 years old, it is probably a good idea to anneal it before using it.

It is not unusual to see original unfired ammo, that is 80 to 100 years old and still in factory boxes, with neck cracks. This happens just from keeping tension on a bullet for that long.
(This is probably further exacerbated by temperature extremes during storage).