Edit:
Since theres been some confusion with dates
In 2016 github made site side searching login only and hid the search bar if you werent logged in. This didnt include searching within a repository so that could still be done, just not all repositories
This year was the change being referred to in this link which made repository level searching require logging in
Blog post: https://github.blog/changelog/2023-06-07-code-search-now-requires-login/
Increasingly, you should be. Search algorithms from Amazon to Google are getting deliberately enshittified in order to force you to see what they want you to see instead of finding what you were actually looking for. For example, things like quotation marks and the minus operator no longer work. I would be supremely unsurprised to learn of Microsoft following suit.