I see the same issue on GitHub too. Recently searched for a Windows API call and neither Google nor DuckDuckGo nor GitHub returned any result.

Cloned a git repo where I thought I remembered seeing the function and 500 MB of downloading later grep confirms that I remembered correctly and that the exact keyword I had been searching for is present multiple times in the source code.

My hunch is that since coders use an ad blocker anyway, it's not financially viable to operate source code search on the public internet.