What does that mean?
It's even more permissive. It doesn't require preserving copyright notices and including a copy of the license with source code forms, like the boost license does.
I'm not a lawyer, but the license uses the same text as the ISC license, but with the text imposing restrictions removed. So I think it should be as legally valid as the ISC license (again, I'm not a lawyer).
The problem with the boost license is it's pretty specific on "works are solely in the form of machine-executable object code generated by a source language processor." being allowed to be redistributed without including a license, so if I was making an interpreted language, or something that would embed it's own source code in the users code (eg a template library or something), then I don't think the boost license would allow them to redistribute that derived work, whereas 0BSD would.
The license is listed on OSI's page as the "Free Public License" for some reason https://opensource.org/licenses/FPL-1.0.0 , but the text is essentially the same.
Here's the whole 0BSD license text:
Copyright (C) DATE by AUTHOR
Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted.
THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
That clause is only about distributing without the copyright notice. If you have any doubts, just include the copyright notice and you can redistribute it.
Disclaimer: I am not a lawyer and this isn't legal advice.
Another advantage Boost has is it is "corporate lawyer approved", meaning the license has never been an issue for any corporation wanting to use D.
I'm guessing that's not what you wanted, because when something is truly public domain then you lose any ability to control it whatsoever, but it is what you asked for.