My concrete example is IANA's tzdb version 2023c. As specified in https://data.iana.org/time-zones/tzdb-2023c/LICENSE the case is clear "if the files date.c, newstrftime.3, and strftime.c are present": The SPDX-License-Identifier is BSD-3-Clause then. However I cannot find any public domain license for IANA or any general public domain license (perhaps with something as general as "This falls under public domain" or something comparable to that) in SPDX' license list https://spdx.org/licenses/. There is a discussion on a forum from eleven years ago: https://opensuse-packaging.opensuse.narkive.com/7ulzl2g8/what-s-the-new-spdx-name-for-a-public-domain-license, but there doesn't seem a clearly fitting solution for me here.
Personally, I see 3 different ways to proceed:
Use some public domain license, already listed as of now that seems like a close match (9 May 2023). Togan Muftuoglo suggests using CPL-1 (https://narkive.com/7ulzl2g8:2.375.36) There are some specific public domain licenses listed: Common Public License 1.0 (suggested by Togan Muftuoglu), Creative Commons Public Domain Dedication and Certification, libselinux public domain notice, NIST Public Domain Notice, Open Data Commons Public Domain Dedication & License 1.0, and Sax Public Domain Notice. Yet, these have very specific individual wording, so I wouldn't use "Creative Commons Public Domain Dedication and Certification" or the others in my case.
Don't mention it at all. On the other side, Stephan Kulow mentions in the same discussion that "As Public Domain is basically the lack of license, spdx does not cover it." (https://narkive.com/7ulzl2g8:3.375.74). So should I just leave it out?
Use a new Public-Domain He also mentions their organization's solution (https://narkive.com/7ulzl2g8:3.528.34): adding a new tag.
3.1. I might also put a "Iana-Public-Domain" for request to be added in the official SPDX license list.
3.2. Also just using a custom and unofficial "Public-Domain" doesn't seem that wrong to me.
What is the best solution?