In May 2022 a conversation in the FSF's IRC channel on Libera.Chat got me thinking that that there may be a benefit to having a version of CC0 that does address patents.
A later development where Fedora began to ban code under CC0 caused me to accelerate my plans to make this public.
To be sure, my general position is that using a strong copyleft license like AGPL-3.0-or-later is best, but the existence of things like The Unlicense show that there's still an interest in such things nonetheless.
A challenge with this is that you don't get one single copyright but many different copyrights from various countries around the world. Even if one area of the world lets you specifically waive or abandon your copyright somehow, that only takes care of one of your many copyrights. If you then ask whether you can also do that with the remainder of your copyrights from all of those other countries around the world, the situation quickly becomes more complicated.
What I've found is that the answer seems to be "no"; that it's not really possible to make something be free of copyright restrictions on a worldwide basis. This leaves an open question about whether things such as The Unlicense can actually operate as intended.
Creative Commons tried to address this with CC0, which was designed to work as both a waiver of rights and also included a broad permissive license as a fallback for the cases where that wouldn't work.
When reading about The Unlicense, a comment on gnu.org is that "If you want to release your work to the public domain, we recommend you use CC0. CC0 also provides a public domain dedication with a fallback license, and is more thorough and mature than The Unlicense."
When drafting CC0 Creative Commons didn't imagine it being used for software and had primarily targeted the scientific data community, which resulted in language being added that patent rights were not granted.
Software patents are an ever-growing concern and so it is important to use licenses with explicit patent grants in them (this is another reason why the AGPL-3.0-or-later I mentioned earlier wins out.) When Creative Commons tried to get CC0 approved by both the FSF and OSI, the concerns over explicitly not granting patent rights eventually resulted in them withdrawing CC0 from consideration by the OSI.
It's still listed at https://www.gnu.org/licenses/license-list.html#CC0 but for being a "Licenses for Works of Practical Use besides Software and Documentation" and a comment that it's not recommmended for works of software because of the patent concerns.
What are people to use if they want to waive their rights (at least, as far as they can)? A choice between The Unlicense, which is less mature and may not operate as intended on a worldwide basis, or CC0 which raises patent concerns by explicitly not granting patent rights?
In my view they should use a strong copyleft license like AGPL-3.0-or-later but if they're not going to then they should have access to the needed tools to put things into the public domain (or at least coming as close to that as they can) safely, effectively, and in a way that doesn't raise patent problems.
Creative Commons says that they make the legal code of their licenses available via CC0, and can be modified as long as it's renamed and none of their trademarks are used so I grabbed a copy of CC0 and worked with three different patent attorneys in three different countries to incorporate language to first waive any patents, and then license them if that is not possible for some reason.
The result is what I'm calling this Worldwide Public Domain Dedication (WPDD).