Re: Tagging of UNCOPYRIGHTABLE material
I worry that I did not describe my use-case well. Let me try again.
Rule: in my organization, all our OSS must have have explicit clearance based on SPDX-License-Identifier and SPDX-FileCopyrightText tags. Our OSS should pass REUSE.software scans without complaints of missing copyright information, etc. If this automatic scan fails, then humans have to get involved, which is annoying and time-consuming for each release.
Therefore, I was looking for the way to add "copyright" and licensing information for uncopyrightable materials that is both correct and passes automatic scans. I definitely want to avoid very dangerous propositions! I perhaps naively thought that claiming an unenforceable copyright was even more dangerous than claiming that something is uncopyrightable. I have no legal training and got a bit worried after browsing https://en.wikipedia.org/wiki/Copyfraud which talks of fines for "false claims of copyright". The chances of losing such a suit would be negligible, but a new avenue for a nuisance lawsuit would be opened -- that was my thinking.
My "proposal" of an UNCOPYRIGHTABLE keyword, which I abandon, aimed to cover the special case of licensing material that could not be copyrighted.
I have learned that the CC Public Domain Mark can be used for the FileCopyrightText and that something like CC-PDDC can be used for the License-Identifier, with a corresponding text file in LICENSES/...