Re: Tagging of UNCOPYRIGHTABLE material
Just to clarify … your question is solely about marking your own works – i.e. stuff that if it were copyrightable, you would hold copyright it?
If so, I would still suggest marking it with CC0-1.0 as explained in REUSE:
1) if it is not copyrightable, you have clearly marked it as such, and the SPDX-FileCopyright tag is merely used as a contact for anyone requiring extra info
2) even if it can be deemed copyrightable, you have released it as CC0-1.0, and the SPDX-FileCopyright tag fulfils all its roles.
If you planning to mark files that other people (would) hold copyright in (if they were copyrightable), than that is different and more touchy question.