Plagiarism is Plagiarism. If you present someone else's work without identifying it as a quote and without attribution, thats Plagiarism, and it must be deleted.

That the motivation for plagiarising is a site ban on certain sources, is no excuse.

So the answer to Is paraphrasing someone else's ideas and not crediting them considered plagiarism? is Yes

A site that bans certain sources has only made their curation harder, and not solved the "problem" of not wanting to hear certain arguments

Plagiarism is plagiarism. If you present someone else's work without identifying it as a quote and without attribution, that’s plagiarism, and it must be deleted.

That the motivation for plagiarising is a site ban on certain sources, is no excuse.

So the answer to Is paraphrasing someone else's ideas and not crediting them considered plagiarism? is yes.

A site that bans certain sources has only made their curation harder, and not solved the "problem" of not wanting to hear certain arguments.