Actually I've been using https://grep.app for ages and while I agree on GitHub I basically only search the repo I'm in, that's mainly because Github's existing search sucks.

On grep.app I regularly search all repos. It's very useful for finding out how to use APIs or where APIs from dependencies are defined.

So I suspect you don't want it because subconsciously you know that Github's "search all" feature won't return you useful results.

Hell they still don't provide a way to filter out test directories which makes the code search inside a single repo useless a lot of the time.