FYI, some of the sites on there that bill themselves (or have billed themselves) as "public domain" actually use proprietary licenses that are not compatible with public domain declarations. I find it very frustrating that "public domain photos" turns up unsplash, pexels, pixabay and a bunch of other sites that have licenses with terms like, "cannot be used to make the model look bad" and "cannot be used to create a competitor to our site".

For many use cases, these licenses are perfectly fine, but they aren't compatible with actual CC-0 declarations (or even CC-BY), so if you are trying to create permissively licensed derivatives, you must avoid them. It may be worth marking those ones with an asterisk or something to avoid misunderstandings.