The notices ILL_PICKUP_READY and ILL_REQUEST_UNAVAIL cannot be deleted by users. Libraries that do not use the ILL module should be able to delete these notices. All the other ILL notices are able to be deleted.
I think we'll need sure that it creates no issues when using the ILL module when the letter is missing. Maybe they were set to 'non deletable' because they are part of the patron messaging preferences? It's similar HOLD.