What I have isn't
exactly what was requested, I think its utility more than makes up for it. Weboas.is (RIP) was an incredibly useful site as it not only integrated regular search, it also lists (mostly free) resources and niche search engines. So while it isn't usually non-indexed stuff, it is basically a catalog of various tools to find whatever non-indexed thing you're searching for. In addition it has things like links to references, resources, and related communities.
Here's one I kind of stumbled on in their news section. If you are the type to find dox, this is essentially a homepage with every conceivable tool you could need.
Unfortunately I also learned about their forums because the "RIP 1992-2022" linked to where the site owner posted their suicide note. Luckily it seems like those left behind aren't wasting any time.
https://weboasis.app/ is a clone made by refugees since the original site is essentially on borrowed time.
https://github.com/weboasis is another way to access these tools. It is far and away one of the best resources I've found in years.