You can fake it by searching one forum at a time with "site:whatever.com", but you have to do one at a time and that doesn't help if there are forums you don't know about.
Google could double its usefulness overnight just by bringing this back.
It is now evident Google is not in the business of making its search any better.
When you are an underdog, making something "better" even for no direct monetisation path, is an obvious win because it all creates more noise and attracts people to your platform. More noise = more business - but Google don't need more noise, they have all the attention they need, they are basically king, they just need to figure out how to milk it, which means the only intrinsic business force is direct monetisation paths... completely different incentives.
Imagine that every Internet user pays just one dollar a month. That's still billions worth of revenue, and it can be segmented further such as basic features available for free and more advanced users (such as large-scale API access) can pay more.
At the moment I'm not aware of any company whose primary service is search. I believe the demand would be there - OS manufacturers currently have to either put up with Google (even if their goals diverge - for example it doesn't work well with Apple's privacy-focused marketing) or Bing.
A third player whose primary business is search (as opposed to using it as a loss-leader to lure people into their ecosystems, like Google and Bing are) could get some use, not to mention that programmatic access to large web index would no doubt be a service many companies would pay for. Think Algolia or hosted ElasticSearch but pre-seeded with a broad index of the web.
At least in my limited understanding of userscript's capabilities, you'd still need to explicitly specify the forums you want to search, but I imagine that would solve lots of use cases for search filters.
I think you can only specify searching one site at a time. With site: search filter.
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Google’s custom search engine’s where you can give a whitelisted amount of sites isn’t very good. At least not any more. It didn’t include lots of pages it’s main index does. Or the rankings were all out of sorts rendering not useful.