God I wish programmers would atop trying to reinvent copyright law. The FSF has done a ton of work examining copyright law in various jurisdictions. Many places have no notion of "public domain"
Personally I would never use or depend on code that has a license its author wrote themselves. It's just too risky and other people will be less inclined to use whatever project I wrote that uses the weirdly-licensed code.
Boost licence
Unlicense
WTFPL
> The copyright notices in the Software and this entire statement, including the above license grant, this restriction and the following disclaimer, must be included in all copies of the Software, in whole or in part, and all derivative works of the Software, unless such copies or derivative works are solely in the form of machine-executable object code generated by a source language processor.
It's similar to MIT or Apache-2.0 in that respect. As for Unlicensed and WTFPL, sure they're both effectively the same as CC0 (though I would personally be cautious about removing copyright notices from a WTFPL codebase).