just our two cents...
When looking at a linux distribution we recently ran into the following package text:
"The contents of this package are ineligible for copyright protection."
We decided to identify this explicitly as license 'none'.
It would be great to have an SPDX-ID for such a case. A tool that matches this text can identify the non-existent license with such id.
One would also be able to mark the own files (i.e. config files) with such an identifier.
Connecting such a case to the public domain appears inappropriate to us. It's not quite what the author intended to say.