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/