Sorry for those of you who don’t want to hear it but if a company like Criterion decides to restore a public domain film and releases it there is no way they can copyright their “restored version”. Think about what restoration is for a second…you can’t copyright something just because you put it back to the way it was supposed to be.
So to answer suspiciouscoffee’s original question yes you can take their restoration of Night of the Living Dead rip it and do whatever you want with it.
Again, it is not Criterion who have copyrighted the new restoration, but MoMa and Image Ten. And yes, it is protected - while the film itself is still public domain, and can still be released by anyone via any of the old masters, the 4K restoration is technically a derivative work that is protected and registered separately, in the same way that someone could copyright their own scan of the film if they chose to do so; the difference is just that your version would be shitty and redundant next to their definitive version. So you can use many of the previous masters for your projects, but not the 4K restoration.