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/
Apparently, this change was in 2016 - before MS bought them. However, I agree with your point. But the proof of that isn’t in restricting search to logged in users. It’s in how they ripped off FOSS code (esp GPL code) for training copilot. They did something that fundamentally damaged the roots of FOSS activity.