Response from Martin Woodward, GitHub’s VP of Developer Relations:
Sorry for the inconvenience @koepnick - while searching across all repos has required being logged in for a long time, when we enhanced the search capabilities earlier in the 2023 we had to extend this to repos as well (see https://github.blog/changelog/2023-06-07-code-search-now-requires-login/).
This is primarily to ensure we can support the load for developers on GitHub and help protect the servers from being overwhelmed by anonymous requests from bots etc.
This has been around for a while. It may be so they can track and throttle LLMs hovering up public code repos.
Either way, it’s a meh. Not sure why anyone would want to clutch their pearls over this. For those who need it, self-hosted gitlab is available.