Very few things are in the public domain unless they are really old, or are fairly old and lacked proper copyright notices at the time first published. I suspect that these things you think are in the public domain really aren’t. If @rider could give some examples of public domain works that have been taken down it might help.
Plagiarism isn’t illegal. You can plagiarise to your heart’s content without fear of it being removed via takedown notices so long as it isn’t copyright.
