"There is an apparently new iOS 18 security feature that reboots iPhones that haven’t been unlocked in a few days, frustrating police by making it harder to break into suspects’ iPhones
…
Apple added “inactivity reboot” code in iOS 18.1 that triggers iPhones to restart after they’ve been locked for four days"
I get that it’s not actually ROM, but this is the normal term used. For example, https://en.wikipedia.org/wiki/List_of_custom_Android_distributions
Emphasis mine.