Thanks to things like the Berne Convention as well as various international
treaties that have come along since then, when someone makes something
they're not getting one single solitary copyright from their home country but
about 200 different copyrights from various countries around the world. It's
not clear if people can legally abandon their all of their copyrights in all
of those countries around the world. It appears that Germany does not allow
this for example. There may be others too.
The reason I mention this is to show that when things like the unlicense
recommend that people use the wording:
"I dedicate any and all copyright interest in this software to the public
domain. I make this dedication for the benefit of the public at large and to
the detriment of my heirs and successors. I intend this dedication to be an
overt act of relinquishment in perpetuity of all present and future rights to
this software under copyright law."
...it like likely won't have full effect on all of those different copyrights
that that person got when they made their contribution i.e., for people in
Germany it's still under full "All Rights Reserved" copyright, since they
don't allow copyright abandonment.
Creative Commons was looking into that very problem and this is why they
wrote CC0 in the way that they did, into 3 parts:
1. The first part tried to abandon all copyrights
2. The second part grants a broad permissive license in the case that the
first can't be done. This addresses the problem with countries that don't
3. The third part is a promise not to sue in case that license doesn't work
for some reason.
And so, something like CC0 is more likely to accomplish the intended goal.